There are a few YouTube videos covering Fred Hanna's technique, but
this one by pipesmagazine (Kevin?) comes closest to the way Fred showed me.
Also, Mark Irwin's valuable yet free compendium of articles called "
A Pipe man's Handbook of Really Useful Information by Chas. Mundungus" covers it well in print.
Regarding moisture level, moister and especially stickier tobaccos are easy to pack too tightly due to their tendency to clump. On the other end of the spectrum are the Tabac Manil Belgian Semois burleys, which are bone dry and are nearly impossible to pack too tightly. With the semois, the challenge is getting it tight enough.
Tamping is nearly as important as packing. Use a light touch and try not to tamp too frequently. If it's burning well, leave well enough alone, but on the other hand, don't wait until it's nearly out, either.