myself if i was already a proficient signguy/gal and wanted to hone my wrapping skills i would pick up the phone and go help at a busy wrap shop, hands on, side by side. i know many ppl who have gone this direction versus the various classes, personally i feel there is more value in a true working environment, with real world issues/situations...
however, i would not recommend this for a noob...a class would be the way to go where you can work at your own pace and get exposed to the basics.
there are some incredible installers in the portland area.
alternatively i have also helped a group of top notch sign ppl who wanted to add 'wraps' to their business get together and bring in a top notch installer to give them a few days of great training...if you can get 6 people together and someone can dedicate some shop space bring the training to you.