Step 1) Look at the various Inq28 blogs on here and elsewhere, there is some stunning work on there and even a cursory glance should give you some great inspiration and a feel for what works (there's really no cut-and-dried method, it's all about it 'feeling' like it works).
Step 2) Decide on a base model. I'd suggest a Warrior of Chaos or a Space Marine (or Coteaz if you can afford to spend a bit)
Step 3) Raid your bits box for anything useful. Even if you don't use half of it, if it looks Chaos-quisitory, grab it.
Step 4) Decide exactly what you're going to do. Having a plan is essential; just mixing and matching is all well and good but there needs to be an element of forethought and composition in there.
Step 5) Build it with blue-
tac. Don't commit to anything until you're certain you're going to like the look of it.
Hope that helps. As for specific bits, I'd suggest a Chaos Demon Bloodletter Hellblade for the
DW, or maybe a
CSM power weapon. It needs to look over-the-top, impractical and painful