When my Dad got a new rim and spokes for his XR600, I figured that I should be able to do it myself with no problem. Looked for a truing jig and realized that they are fairly expensive. So I started to look around the garage to see what I could use that would be square.
Ended up using a pair of axle stands, two quick grips to hold the axle in place, a wooden match, some wire and duct tape. Then set it on top of a table so I could easily work without bending down and being uncomfortable.
The wooden match was used to gauge the lateral runout (side to side wobble). Just tape it on there so you can adjust it to get closer and gauge how to adjust the nipples.
For radial run out (up and down wobble) I used safety wire. When it reached a low spot, I heard it scratch a little bit, since it isn't a nice finish I didn't care about scratches.
When I was finished and I had absolutely no wobble left in the rim I mounted it on bike and spun it again with perfect results.
Best advice is to take it slow and make minor adjustments. Might take a little longer, but small adjustments are easy to correct and big ones really screw you up.