Pick your powers in level-up order.
Slot things out.
If you need to shuffle things around, you can actually drag powers around to swap with another power. Say you wanted to take KO Blow from Level 20 to Level 30, you simply click, hold and drag. Now if the power that's in L30 isn't available at L20, it'll immediately turn red to let you know the build is illegal.
You have the tab group in the lower left-hand corner to give power info and your general total for things like Defense, Resist, Recovery, Regen and Recharge.
If you want more info, click on Window --> Advanced Totals
To get optimal placement for your build, once you get it built, click on Slot/Enhancement --> Slots --> Auto-Arrange. It basically packs the slot distribution.
This way, instead of having an L50 slot in a L32 power, you get L33-34, etc slots. This way, if you are re-working a build, all your high-level slots go to the last powers you pick.