So, different armies have different styles of conversions.
Orks are easy, and fun. You just take an absurdly large bitz box, get a shape in your mind, and get creative with it. Its more art than anything else.
Chaos, is alot of mutation. You break out the sculptin gear and have fun making tentacles, pustules, claws, weird muscly stuffs. Its pretty forgiving, as theres no set way something has to look. And any mistakes can be covered pretty easy.
Nids are similar to chaos, they mutate too, so some simple & clean sculpt work can go a long way.
Now, the humans. This is where things start needing to be a bit cleaner. Sure, you can use battle damage and weathering to cover alot of of stuff if the convert isnt great. But, as Ive been learning with my Hellhound armored car variant- keeping clean symetrical lines is not easy.
Im pretty sure my next IG project will have some differences. First, Im going to acquire a set of sliding calipers. This will let me measure a spot and fit in pieces exactly, rather than wasting time getting it close, then filing/cuttin it down to fit. As an example, with some sheet plastic, and a set of calipers I could make a set of IG track guards in about 5 minutes. With the stuff I have now- probly 20 minutes and may, or may not, come out symmetrical/even.
For small projects this isnt an issue, theyre easy to adjust/compensate on. But I'm looking at getting into some more advanced kit bashes. I hate the basic chimera & leman russ hull designs. Ive been lookin over several ways to update them- much like they did with the MkII Rhinos and Land Raiders. They kept the spirit of the previous models, but 'updated' them in a very pleasing way. Why GW didnt take the opportunity to do that this time around is baffling. Until you look at sales- the clunky old models are still selling well- no point in replacing what still sells. So, if my ideas take form as I hope, I'll be able to either create a conversion process for the existng models, or something of my own creation that will look different, but still be recognizable as an 'upgraded' version of the original.
Another project idea- will be kit bashing some Valk/Vendettas out of some much more affordable Hind 24 gunship models- which are actually an almost exact equivalent in terms of tactical usage and effect. And the 1/48th models are almost spot on for size (minus the wings).