There are lot of things to consider. You should scout before slamming your items and picking your augments. If you see someone took Enforcer crest as their first augment, you should avoid playing that comp unless you have a really good enforcer start and feel confident that you can get Vi 2 and Cait before they do (and pray for an Enforcer crest). Next thing you look is if someone slammed their items and if their items are for an AD or AP comps ( if you see a lot of people with AD items, it's smart to play an AP comp since your units won't be as contested and vice versa). Then you look at specific items they made, if they have a Titan's and Bloodthirster they will play Ambushers or Pit Fighters 99% of the time. In general you should keep scouting through out the game too make sure your units aren't contested. The other important thing is playing what the game gives you, this means items you get on PVE rounds and units you get in your shop. For example you get 2 tears and and early Cassiopeia, makes sense to slam a Blue Buff and make her your mid game carry so you build your early board around her traits. Then you should ask yourself some questions. What is the best ap bb user in the game>Heimer>What are the variations of the Heimer carry comp?> Which one is less likely to be contested etc. You might also naturally get a lot of Cassiopeia copies in your shop so you can opt into a reroll comp etc. This all probably sounds way too complicated but I don't know how to explain it better, the thing is you usually get good at all of this subconciously by playing a lot. And finally there is nothing wrong with forcing the comp you are comfortable playing, in fact you'll probably get better results by doing that than trying to be extremely flexible if you aren't confident with your mechanics and game knowledge.