You can make anything into a ladder. even something slippery like a plastic fish.

Now if you want to make one of the ladders you see if hl, often that damn red one, select the ladder, and make a ladder, but how you ask, well once you have your ladder ready and waiting.

Make sure its not really thick as it will look silly, (bit like me) now select the aaa texture and put it over your ladder, so you can see the aaa texture in front of your ladder, the same height and width, a mirror of it, now select the aaa texture (pink with a's on), make sure its not massively thick please. and make it into a func_squirrel, no wait a func_ladder, and then you have it. go test your squirrel, er I mean ladder, and adjust it as you see fit.

In other words:

>Make a brush with the ladder texture on it, a few units thick. then align your ladder texture on it well.(make this brush a func_wall or func_illusionary and set render mode solid,fx amount 255)
>Produce a block with the aaa texture on it and place it where you want the player to climb, in most cases on the front of the ladder, like a sandwich, with the aaa texture on top or in front, ok like some toast then.
>Make the aaa texture into a func_ladder
>Test your ladder and wash some windows