Because, I see many people wanting to know how to do this, so Ill make it short and simple.
Step 1 = Look for a ladder.
Step 2 = Approach the ladder and climb up a bit
Step 3 = While on the ladder, hold the "W" key and the "D" arrow key at the same time, look up the ladder, and watch yourself climb it. (Climb the ladder basically while holding A or D arrow key.)
Step 4 = Watch as you go up the ladder 2 x faster.
Credits to me,
I hope to help you :D