Can you use in winter

Yes, they do work in the winter time as well as the summer time.

Can I drive with the wiper saver on if it isn't raining?

Yes, as long as you don’t need to use your wipers. Bottom line is “you can” but why would you want to. The savers are easy to put on and take off and driving with them installed increases the chances of you losing one or both while on the road.

How do they go on ?

One end is stitched closed, the other has velcro. The wiper blade slides in the stitched side with the other side secured with the velcro. There is a room in the middle for the arm. Fits pretty well.