This can be done quite close to the start of the game.
Head up to a mountain viewpoint called "Fannaraki Mountain"
Its a top this big, beautiful, snowy mountain top and can take a while to get there.
Once here, on the video you will see the location on the map, its slightly south east of the viewpoint.
This is the ONLY place in the game where there is enough snow to slide down for over 150M.
If you actually place a cursor at the bottom, you can see how long you have actually got.. Usually around 175-180M.
Be aware, this CAN be glitchy and buggy.
If it does, try to hard reset or cold rest the console.
Quit out of the game and attempt to get back into it.
Try holding B and controlling Eivor.
Try hitting B once to slide, then letting go.
Trying all different things could be key to unlocking it.
Hope it works for you!