Overtrousers go on over the trousers you are already wearing, usually without taking your boots off. That is the whole point of them, and it is what separates them from a proper waterproof trouser.
Overtrousers, chaps or waterproof trousers?
If you are out all day in the wet, buy a waterproof trouser and wear it. Overtrousers are for the day that turns — you carry them, and when the rain comes on you pull them over the top. Deerhunter's Hurricane and Strike Extreme pull-overs and the Arctic Storm overtrousers are cut for exactly that.
Chaps cover the front of the leg only. SwedTeam's Crest PU leg chaps and Jack Pyke's waterproof chaps are lighter to carry and quicker to put on than a full overtrouser, and they suit sitting work and walking through wet cover where the soaking comes from the front.
Waxed cotton
Hoggs of Fife's waxed overtrousers and waxed treggings are the traditional answer and still a good one in thorn. Wax is quieter than most laminates and shrugs off bramble that would eventually tell on a lightweight shell, but it needs re-proofing and it is heavier to carry.
Check the leg zips
The single thing people get wrong is buying an overtrouser whose zips do not open far enough to go on over boots. If you expect to put them on in a hurry, in a wood, standing on one leg, look at the zip length before the fabric.
See also gaiters and waterproof clothing.
