The beginning of the stairway is at around 0x, 0z. That's to the left of you if you face the river and bridge. The river practically goes all the way to the entrance. It's about 150 total sneet away from spawn, so it's a bit of a travel, but that's okay because that will surely be the longest part of this!!

So you're traveling to Sneaven, either walking or flying. Both work, though one's certainly faster. If you're wondering how long this'll take, and wondering how high you need to stay to not hit the ground at mach 1000 or to not overshoot if you're talking the latter method, don't worry. I had the same concern, so I made this guide.

The coordinates of Sneaven (accessable in the esc menu) are: X = 0, Y = 3000, Z = -4050. .

Welcome to Sneaven. Live long and prosper