Easiest way is to connect the TX to the RX and vice versa.
Nothing else is needed. Just use the standard Serial stuff.
How does the power work? Do I connect one through USB and the other through a power adaptor? Or do they both run off a shared connection (if not, do they have to have a common ground?).
Which data type is best for exchanging values from 0 to 255, DEC?
Cheers,
Sascha