Haven't done an M104, but I've done (or tried to do) a M103 twice. Generally seems to be that, of the four bolts securing the two mounts, three are pretty easy, and one is very, very hard.

On the bottom are either 17mm bolts or 8mm (?) allens. The allens are easy to round, so be careful with them. May want to shim up your allen socket with a little scrap of paper towel to take all the slop out. Also, if you're not careful, it's easy to lose a bolt in the crossmember. It once took me a couple hours to fish one out of that situation.

May want to have a couple of extra bolts on hand in case you round or lose one.

Also, soak the bolts in penetrating oil a day or two before you attempt disassembly.

On top are 17mm bolts. For the passenger side one, I put a box end wrench on the bolt, and levered a bar against it and a SOLID, structural part of the engine bay. For the driver's side, I did a combination of the above, plus lots of penetrating oil, and tapping on a wrench with a hammer. This one is the hard one, mostly because there's no clearance to get any kind of leverage on it.

If you're careful, you can lift the engine from the oil pan, by using some 2x4 blocks of wood spread along one edge of the pan and a floor jack. Jack up only one side at a time, and be careful of the clearance with the fan shroud and other engine bay items.

Loosen all the bolts, lift and replace one side, start bolts (but not tight), lift and replace the other side, torque bolts, and that's pretty much it.

Use a little anti-seize when replacing bolts. I forget the exact torque spec, it may be around 30 Nm.