You can get the info for running theme forces in the forces of (insert faction name here) or in a few of the latest no quarters (Did they start in #32?).
What they will do is restrict you to the units listed at the top of the theme list and this gives you the tier 1 benefit straight away. Then you can build your list and if you meet the tier requirements you gain that listed benefit but you have to meet all the previous requirements as well. So for a tier 4 list you have to meet the requirements for 1,2,3 & 4.
As an example:
Mortenebra, Infernal Machines (
link to a preview)
If I restrict myself to the things listed along the top I can bring more necrotech's (repair warjacks) and scrap thralls get advanced deployment (anything getting you closer to the enemy is generally good).
I put in two warwitch sirens then my big warjacks are had to shoot in the first round (picture says turn but this was errata'd) That's tier 2 down.
Tier 3 I just have to forgo units and those big heavy warjacks are a point cheaper.
and finally if I have 4 of those cheaper, harder to hit in the first round, easily repaired with necrotech's, big warjacks then I get to place them 2" further forward when deploying them.