I use one of these guys to turn the part while threading:
https://littlemachineshop.com/products/product_view.php?ProductID=3897&category=
Seriously one of the best accessory purchases I've made for my lathe.
My approach to threading on the lathe is as follows:
1.UNPLUG lathe
2. Insert crank in the back of the headstock spindle and tighten
3. Insert tap or die holder in tailstock Jacob's chuck.
4. Make sure tailstock is not locked down, and can slide freely.
5. Gently butt tap or die up against piece to be threaded, and turn crank counter clockwise a few times to make sure the tap or die is nicely centered up on the workpiece.
6. Turn crank clockwise and thread the work piece, while simultaneously applying enough pressure to move the tailstock forward as the tap or die gets pulled into the work piece. Back off the tap or die off a bit after every couple of turns to break the chips loose.
This has worked very well for me. The key is to move the tailstock with your right hand while threading. Don't let the movement of the work piece pulling in the tap or die move the tailstock, or that could potentially lead to some nasty breakages.
-John