Part 3: Draft a Complete Build
At this point, you should understand how to evaluate individual skills as well as your base model. The next step is to combine all of this information to create a cohesive and powerful build, which is very difficult and requires quite a bit of practice.
'The Build'. Stun, survival/positioning skill, something strong, teamfight ultimate. This is a viable blueprint for maybe 60% of games- it works for both supports and caster/right click carries. The number one mistake that experienced AD players make is overcommiting to a single theme in their build. This could be taking 3 or even 4 right click skills, or 3-4 nukes, or a bunch of tanky/survival abilities. The basic issue is that all of these builds have major flaws with nothing to cover them. Your 3 right click modifiers don't matter if you get bullied out of lane and fall behind in levels and farm. Your 3 nukes get made irrelevant by a Pipe of Insight or BKB. Who cares if you're incredibly tanky when you can't deal any damage or disable the enemy team- they can just kill everyone else and then 4v1 you. The only exception is the All Stuns support build (which can very rarely be a caster carry), which basically only works because stuns are really, really good.
Oops All Cast Range: My Favorite Support Build. The basic idea here is a support build that takes the best spell with a cast range, preferably AOE stuns or just any disable, for every single one of your picks. Then, you build Arcane Boots -> disassemble into Aether Lens and Tranquil Boots. The Aether Lens makes all of your skills stronger, and because of all the disable you have you're very hard to kill except by someone who can burst you down before you can do anything (which often means both a positioning tool and a BKB/Orchid/hex). I play this in probably a quarter of my games. It's good.
Drafting Synergy. The first step is to figure out whether it's even worth trying to make a synergistic build at all. Most synergy builds have a 'payoff' skill and one or more 'enabler' skills. Payoff skills are strong, but only when certain conditions are met, and enablers fill those conditions, possibly at the cost of being individually weaker. Sticky Napalm is a payoff skill (if you deal many instances of damage, then Napalm does a lot of damage) while Rot is an enabler since it deals many instances of damage. The issue is that you need both of these in order for the synergy to work, and even then, a Crystal Maiden would probably just rather have two stuns than a build that deals damage to her already fragile model and requires her to be up close. The next step is to consider the fail case, where one of your pieces gets counterpicked. Basically, if you don't get counterpicked, you get both skills no matter which order you draft them, so you should always assume you will get counterpicked and pick the more independent skill first. This is usually the enabler skill, but can sometimes be the payoff if there are several enablers in the draft. A huge mistake I see frequently is people taking Counter Helix before Berserker's Call- the former is basically useless on its own outside of early-game farming, while the latter is an AOE stun that pierces Spell Immunity, which is crazy strong both early and late. Part of why Aftershock is so strong (besides being an AOE stun) is that basically every active skill in the game is an enabler for it. In general, I think synergy builds are significantly overdrafted, so I would advise going for them significantly less often than your instincts tell you.
Drafting Right-Click Carry. The first secret to right-click drafting is that positioning skills are the best right-click skills. Your right click only matters if you actually get to attack people. Anti-Mage has Blink, Juggernaut has Omnislash, Faceless Void has Time Walk and Chronosphere, Slark has Pounce, Phantom Assassin has Phantom Strike- all the best right clickers in normal Dota have a way to get on top of people so that they can attack them. The best part is that many of these positioning skills are useful even outside of carry drafts, making them much more reliable first picks. The other secret to being a carry is that you have to not die a bunch early game, or you'll get too far behind in farm to matter at all. So, when drafting right-click carry, priority 1 is positioning, priority 2 is surviving early and farming, and only priority 3 is actual attack modifiers and damage.
Drafting Aghanim's Scepter. The idea here is to pick multiple abilities that get upgraded by Aghanim's Scepter, then buy Agh's and have them all upgraded. My advice here is simply to not even think about Agh's synergy until the 3rd round of drafting. Why? Well, simply put, 4200 gold is a lot of gold, and you can very easily lose games (or get to an unwinnable position) before you get to buy it and be useful. So it's better to draft a solid foundation first, and if you find in the 3rd or 4th draft round that you already have one or two Agh's abilities and can buy another, go for it. Don't take Starstorm or Decay in the first round of the draft. Just don't. They're not worth it.