Brr. Leg warmers are OK for me down to about +5C; I'll go with tights below that, and heavy tights below -5C.
To respond to your original question, you want to roll your bike shorts up, pull the warmers up, then roll the shorts over the top of the warmers. The part of your shorts that grips your legs will then help hold up the warmers.
(In my case, then pedal a mile, stop, and re-adjust the warmers and shorts. Another reason I like tights when it gets cold!)
Leg warmers are useful in changeable conditions such as mountain climbing and for rides with big temp differentials such as some AM/PM commutes.
If you are going to start and finish the ride in the same clothing (as you would at -10C) then you will be far more comfortable in full leggings or long bib tights. Wear them over shorts to give 2 layers of protection to stop your tackle from freezing.