first just get it on a stand, or hang it by the saddle, and turn the cranks. if it's skipping up or down without any shifting, turn the barrel adjuster on the rear dr the opposite direction to which the chain is skipping until it stops. continue turning the cranks and twisting the adjuster until noise increases, then back off a little till you find the quietest sweet spot.
if that doesn't work, it may be time for a new chain (it probably is anyway, i'm just guessing) but often if the skipping can't be corrected by proper rear derailleur adjustment it's time for new chain/cassette combo.