my cat can't jump up onto the counter, and we have dogs who steal her food if its on the floor......so I created a ladder so she can climb up onto the counter top to eat. 

the supports are 20mm square aluminium tube, which is quite easy to find at a hardware store

Print as many steps as you need, plus one each of the top and bottom parts. There are 2 types of steps, with and without mounting brackets, and a base for the floor. Position the steps equally along the length of the support, drill 4mm holes and rivet in place.

 PLA works just fine, I used 60% infill for strength, my cat is about 6kg

EDIT 1 June 2022: Updated the mounting parts for more strength. Yes, either my dogs or cats managed to break it!