This guide explains how to slide on your shield pretty well:
Let’s explain how to surf on the shield. To execute the shield surf, stand on any slope that you want to go down. Then, hold ZL so that Link performs the shield block. Then just press A (or jump and press A) and you’ll be on your way down.
As we’ve said above, during your surf on your shield, you can do a 1080 spin by pressing Y (works as a Spin Attack) and you can apparently perform tricks by hitting X and tilting the left stick at the same time.
A word of warning, however, from the same guide:
Surfing on the shield, any shield, damages it significantly. Try and avoid going down rocky terrain and stick to snow and/or grass. However, even that won’t save your shield for long. Be especially careful not to let the shield [break] while you’re mid-surf, since that will propel you forwards and deal damage or even send you careening down the mountain to your death.
Source : Link , Question Author : AdrenalineNerd , Answer Author : Community