Make sure to check the other build as well, is just as important as this one or maybe even more (if you need some defense).

With SAW, 90% of the game you are defensive, usually only farming(make sure to get the last hit on minions btw, gold is really important), and also controlling space, thanks to your high damage since the beginning.
With SAW, you should always position yourself well, since your movement is limited, and any kind of negative effect will basically kill you if you are in the wrong spot. Do NOT take risks. You should be offensive only when a teammate is being targeted, or if they are attacking someone while using stuns, roots or slows to prevent the enemy from escaping, so use your absurd firepower to control space. Do not allow anyone to stay on your range, make them back off by shooting them(high damage) or get away from them if you dont have enough fire power to deal with them; be defensive with SAW, make them come to you, that's the golden rule, that's why SAW can demolish objectives faster than any other carry in the game, and has the WORST movement when compared to other WP carries. He's an interesting hero, since he has to have great positioning, and requires the player to have good sense of danger, because if the enemy gets in your range without you being ready and stuns you, you are dead. Stay with your captain whenever you want to be offensive, so he can scout bushes for you, and also save or life with Crucible or War Treads.
Once again: Protect your teammates whenever they are being attack with your firepower, and use their stuns to melt the enemy heroes while they can't get out of your range. Protect your towers by controlling space, and be ALWAYS alert for damage, since you cant low them down, stun, or use any kind of mobility skill(there's the knife, but it can't deal with slowdowns unlike Baron and Gwen, and most importantly, it removes all the stacks from SAW, so he loses his ability to control space, therefore he won't be able to protect his towers/teammates).

It's popular belief that SAW(because of his low mobility) shouldn't bother about moving around that much and should be stationary;that his movement is "useless" and that you should usually look for trading kills since you cannot run away. But that is WRONG. A good SAW player always moves around while shooting, because any kind for movement is valuable for a slow hero like him. A good use of basic attacking while moving can allow him to get 1-3 more shoots compared to when he's just stationary, which is HUGE, since he has terrible mobility for chasing targets. Using that method can save your life a lot, you can reposition yourself while shooting, get away from dangerous areas while shooting, and sometimes even being able to evade a skill just because of that small, but valuable movement; that's usually one of the things that separate good SAW players from bad ones; movement is key. Just like any carry, SAW only goes stationary in the worst situations possible, only if he wants completely optimize his damage and there's no way of escaping.

The full critical damage build is really powerful, not only it has insanely high damage(to claim objectives and keep enemies in check) but it also has great attack speed, and attack speed is GREAT for SAW, since it allows you to get the initial stacks faster, so you are always ready do deal max damage anytime, even after escaping or taking a break from the fight, that way it's harder for them to attack you while you are vulnerable (low stacks). BUT, you need to understand that this build is extremely weak against enemies that have multiple heavy stuns on their team(like koshka, tony, churnwalker,reim,or anyone that has a heavy stuns). You can should this build if they have ONLY ONE heavy stun hero or less(since your captain can help you out with a Crucible) otherwise, you will die thanks to stuns and won't be able to shoot even once.

Against RED or HIGH YELLOW threat levels I don't recommend this build, unless there's only one of them. In case of two or more heroes of high threat level I recommend either you buying an Aegis(to help your captain with the stuns) or a Celestial Shroud(against heavy CP damage like mages or CP junglers)
Also, I didn't put the rest of the heroes because there isn't much to say about them... The build probably does fine against them for the most part

Any hero that doesn't have a great mobility, that has no significant stun ability or lower range are generally a low threat for SAW,only if you don't play into their strengths of course. But the general rule is: Stay away from slow heroes, they will usually kill SAW in a 1v1 if they are constantly attacking each other, even if SAW has probably the highest DPS in the game, he's still vulnerable to junglers like most of the carries.

SAW is known for being very powerful against anything that gets inside his basic attack range, but he is also extremely vulnerable against anyone that can damage him outside of his range(mages) and he's also vulnerable when 2 or more enemy players try to attack him together, since he doesn't have a lot of move speed and doesn't have an extremely reliable way to escape. For that reason, you should ALWAYS start with the 1st ability on 3v3, it allows saw to get away from early team fights and also can be used for last hitting the tree on the middle for your team. The book not only is a initial item for serpent mask, but is also really important to give SAW that some health back, since he's usually vulnerable against mages and also has naturally a high damage(helps with healing)

Serpent mask: Well, serpent mask is a MUST for SAW. As said before, he is slow(susceptible to take a lot of damage) and has VERY HIGH damage(susceptible to heal A LOT). That's a key item for saw, should be his first one because it's the reason why he survives in the early game and can defeat his enemies during late game.
Halcyon Chargers: SAW has the lowest energy in the game, which means that ANY energy item will give him enough energy for the rest of the game. That's why Halcyon Chargers are so good: Almost unlimited energy(for SAW), cooldown reduction (which is very useful in general) and also the best individual boots in general(since Journey Boots have been completely nerfed). Usually better than War Treads because SAW is very slow and has to use boots often, either to chase heroes or run away from them.
Bonesaw: Excellent item for SAW, it's vital for taking down objectives, shredding enemy armor(they will probably buy it so bonesaw increases the damage a lot) and also gives great attack speed which he desperately needs for versatility, you should do it as your second damage item, because it's great for taking down gold miners, towers and helps with damage and heals(serpent mask)
Monocles: after building the key items for SAW(the ones above) you can start increasing your damage, and since monocles are expensive and don't give a lot of benefits with only one of them, you should NEVER start with them; your damage will be low, no attack speed, no healing, no guaranteed critical... It's terrible. But when you already have a solid damage with the key items and have time to build them while helping your them, they are really important. Your damage will increase a lot, way more then with any other item, so this is when SAW really starts becoming an offensive threat(since you as SAW are really defensive on early game, mainly controlling distance).
Tornado Trigger: Tornado is great for SAW as a last item, since it gives A TON of attack speed, movement speed, and also guarantees critical hit(2 monocles+1 tornado=100% CRIT rate), now you are faster, your damage is CRAZY and you are way more versatile then before.

That one is in case they have 2 or more high thread level heroes on the enemy team, in that case you should use a defense to protect SAW and make your captain's job easier by buying Aegis or one other specific defense against the main threat. Your damage is lower, but you have a way higher chance of survival and you can still help your team...
It's way more helpful than the other one if they have a lot of stuns on the enemy team.
Keep in mind: Since you buy Aegis, you will take longer to deal heavier damage, but it's better to be alive and holding objectives and helping teammates then dying instantly or staying too far from your team because of the fear of being locked down... Don't worry with enough patient you will because a great carry on late game, and also a good defensive carry on the early game.