Digitrax makes an auto-reversing unit called the AR1. I have used this and the MRC A/R module. They both cost about $24 and both work well. Simple 4-wire hookup.
Tony's Train Exchange also has combo circuit protector and A/R module for a little more.
See the Digitrax site for detail and the instruction sheet, I think.
Nothing to it. The loop must be gapped and insulated from the main. The A/R unit attaches to the bus or the main and feeds the loop. That's it. Another great thing about DCC.