Russell Rainey, tele-binding maker, has this advice for alpine skiers seeking to make the switch: hire out a good set of modern gear (plastic telemark boots, shaped skis) and start out just doing the same parallel turns you know how to do. Then when you are feeling confident having a free heel, you can try a telemark turn when you feel like it.
If I had any advice regarding technique:
1. Weight equal on both skis, not focussed on the outside ski;
2. Keep upper body very "square" to the fall line, this brings on a counter-rotation in the hips that lets you weight those skis equally. Some people feel themselves leaning their inside shoulder toward the tip of the outside ski to achieve this.
The learning curve is easy for experts because they know what controls their skiing - they just make the necessary changes and go on. Intermediates can find it hard because they can't break habits - eg weighting the outside ski. The real problem is they don't know what to change, so lessons will help a lot!
Have a look at www.telemarktips.com
for much more good advice.