Practice???

Where are you? Alaska Fly Fishers have free regular tying clinics through the winter in Anchorage, but they are done for now. Mossy's Fly Shop, also in Anchorage has tie-ins on Thursday evenings and there are lots of folks there that can show you how to handle dubbing very neatly... But if you are in Fairbanks or elsewhere that is not going to help much.

A couple rules of dubbing:
1) Start with exactly as much as you think you need and put half of it back.
2) Keep the dubbing thin on the thread, just barely cover it, and make more passes to achieve bulk. That will teach you far better how to regulate your noodle.
3) Scuds are too small to worry about anything like dubbing loops or other fancy stuff... A Nor-Vise would make your life easier though...

3/8" is plenty long enough to cut the yarn... 1/2" is too much IMO&E.