Level up on 2 if you are strong and want to start a win streak.
Level up to 5 after the carousel on Round 4 if you want to continue your win streak.
If you were lose streaking up to Round 7, you can level up to 5 if you don't think you can beat the creeps.
Level up to 6 if you are on a win streak during Round 2 or level up to 5 if you haven't yet.
Win streak or lose streak, you should normally catch up to level 6 on Round 2. Roll a small amount if you are very weak and need to stabilize.
Before the fight on Round 1, level up to 7 and roll down if necessary.
7 After raptors, level to 8 if you haven't already and roll down to complete your team. If you can stay strong without rolling too much, start saving up for level 9 to add in Thresh/Urgot combo.
In the early game you should run whatever units you find. You will need early game damage in order to win fights.
Once you're in the midgame, you can start building your 4 Vanguard frontline with Cassiopeia doing most of the work.
This comp requires a good start on Casssiopeia as she will be the main source of damage with a defensive oritented team.
Look around your lobby to see what other defensive items you should try to build.
With 6 Vanguards, you will be very strong against attack damage teams, so be wary if there are too many Sorcerer teams and go for 4 Mystic instead in those cases.