Hi and Welcome!
I have always taught the stairs by putting them one step from the bottom and then holding a treat just out of reach, so they have to come down to get it... then move them up two stairs and do it again, etc. etc. Then I do the same thing in reverse for going up.
If the treat is really yummy and worth it they WILL come down and get it
It has usually only taken a couple of tries and they were running up and down the stairs like they always knew how to use them!
Good Luck and Welcome!