Renderbump is not too difficult to use, once you get how it works.
I haven't used it for a very long time but here is as I remember it. If I miss something, I'm sorry in advance. But this should be pretty much how it works.
Export both low and high poly to .ase or .lwo file. They must occupy the same space. You must export normals and UV info. Although UV is only used for the low poly.
Put both exported models in base/models
This location is not mandatory, so feel free to use your own location.
Make sure you have a Doom material that actually works for the low poly model. If the material does not work the renderbump will not work.
Then you need to add a line like this...
renderbump -size 512 512 -trace 0.02 -aa 2 models/new_local.tga models/highpoly.ase
... to your material. This is the info to create the proper normal map and reference the high poly model.
I recommend to run Doom in windowed mode before rendering the map. The size there is the size of the map it saves out. The "aa" is anti-aliasing and I'll suggest to make it 0 for testing purpose to speed it up. Once you get a good result you can push it up for final renders. The trace is the distance it will trace for high poly normals. Tweak this if you are missing some normals or get some overlaps of close geometry. The next line is the file name of the TGA it'll save and the last bit is the reference to the high poly model
So then inside Doom you bring down the console and type..
This of course needs to point to your low poly model. You can use TAB to just quickly pick through. I'm sure you know the TAB shortcut in the Doom console.
So that is pretty much it.
Main things are to have proper exports. Just to be safe you can keep them scaled in game dimensions. So it's not like super huge or really tiny. And of course the material has to work. I know lots of people struggle with that, not using material names correctly and so on.
I hope this is helpful and I didn't miss something important..