Don't really need a detailed writeup, it's not a hard swap.
Things needed from the M40:
Engine mounts and arms, sump, dipstick, spigot bearing, gearbox, airbox.
Radiator - I used my M40 one but you may have to use a larger rad from anything that fits (M20 one maybe)
Tricky bits:
Exhaust manifold will need cut and rewelded, it is stainless so this is doable. Downpipe will also probably need cutting and welding.
Airbox will need a bracket as will the coilpack.
Loom won't fit very well as the ECU is on the opposite side on the e36, you can cut this to get it to fit if you are confident in your soldering. Some wires you won't be able to cut/splice as they are shielded.
Lift old engine out, leave gearbox inplace or not depending on your preference.
Transfer sump over to M42 engine, if serpentine then you'll need to bend the dipstick tube to clear the belts and make a retaining bolt tab to hold it inplace. Change spigot bearing in the centre of the flywheel, also change over/select best clutch+flywheel assy you have, replace the dualmass on the M42 if it has it. Change starter over if you swap flywheels. Fit mount arms off M40 to the M42 block. Take exhaust manifold off M42.
Put engine in, check that it fits ok. Fabricate the exhaust manifold/downpipe so it fits in and doesn't hit anything (steering guibo). If it's a serpentine engine then bend PAS pipes and attach to e30 rack. If its v-belt then change pump over from M40 with e30 pipes.
Loom wiring: what I did was cut the C101 off the M40 engine loom and took the X20 from the e36 body and used these to make a go between cable, this means I didn't need to chop the e30 car loom or the e36 engine loom. Wiring diagram here:
http://www.e30zone.net/modules.php?name ... highlight=
Mileage may vary, check, double check and check again before deciding it's right, each engine/car may have small changes. The EWS wiring is the bare minimum needed to get it to work, doesn't include the ignition cutoffs.
Throttle cable I think is too long, I chopped mine down and clamped it at the correct length.
Can't think of anything else, I'm sure there is other stuff but most of it's pretty simple. The stopper is the exhaust manifold if you don't have access to a welder. Any questions let me know:)