Maxwell;
Assuming you are working with 1/35th scale items. You could check Verlinden. If you wanted to make your own, it is quite simple. I make items such as this the same way they appear in real life. To begin, measure a simple staircase in your house or of similar height. What you need is the height of the stair in relation to the next up or down. The width is determined by you. With your stair measurement reduce it to 1/35 scale and draw a staircase with the scale measurement of step height. You end up with a zigzag pattern all that is required is adding the 90 degree angle along the bottom and rear and you have the profile of your stairs.
With the profile of your staircase cut out (make two) you can lay out the width of the staircase. Then it is just a matter of adding the steps across the cutouts and adding your wall to one side.
I hope this helps. If you lay it out on paper you will understand what is hard to put into text.
Cheers;
Gregory