If you want to just go punch holes in the sky, right and downthrust are probably not necessary. But for those occassions when your trying to correct a bad habit, some designs do infact benefit from one or both. For example, my GP CAP 232-40 has 5 degrees right thrust built into the firewall, and I added some downthrust to improve its flight.

Adding right or down thrust is as simple as adding some washers behind the engine mount, assuming your engine mount is screwed into the firewall. Its not a precise science. Try one or two washers, and if it improves, then try adding more. Find your optimum and then have fun.

As for off center, if the plane is designed for right thrust, then you will want to put your engine mount slightly off center (you'll have to figure out the distance) to keep your propellor on center. If you don't, however,you probably wont notice the difference. Its only a fraction of an inch.

Good luck,
Brad