There's not a whole lot to it, and there are many different ways -- I'd suggest going to a street rod show and look at how those guys do motor mounts. You can get some of the parts you need from rod part vendors as well, and make the rest. I made a piece that bolts to the bolt pattern on the block, made "towers" coming up from the crossmember (and braced to the sides), and made it so you can bolt the two together with a cylindrical polyurethane bushing in between. Make sure you get the engine angle right.
It's kind of hard to describe, but if you go to the next available street rod show in your area you'll see tons of great examples.