Stairs are one of those things that look complicated, but are actually relatively simple to make. Carpenters build stairs all the time with little difficulty. There is no reason model makers and hobbyists cannot do likewise. Correctly proportioned stairs follow the International Code Council recommendations, that the tread or unit run be at least 25 cm (10 inches) deep, and the riser at most 19.375 cm (7 3/4 inches) high. Your paper stair measurements need not be that precise, but for scale models, adhering to those proportions will help the model look correct.
Decide the unit size for your paper stairs. For example, if you want to maintain the proportions mentioned above, but in a small scale -- say 1/10 actual size --you would make your "unit" measurement 1/10 of a cm multiplied by 25 for the unit run length (or tread), and by 19.375 for the riser. In other words, each stair would be 2.5 cm (1 inch) deep by 1.93 cm (3/4 inch) high.
Cut the heavy paper into a square of the desired size. For this example, cut it into a 7.5 cm by 7.5 cm (3 inch by 3 inch) square, which works out to precisely 5 steps, but you may use any size you prefer.
Start at the lower left corner and draw a line straight up 1.9 cm (3/4 inch) then at a 90 degree angle to the right 2.5 cm (1 inch)to make the first riser and unit run. Repeat the action---up 1.9 cm (3/4 inch); over 2.5 cm (1 inch) -- until you reach the top right corner. You should have five drawn steps spanning the entire space from one corner to the other.
Cut out the stairs carefully so that you have two identical sets of steps when you finish cutting the sheet in half diagonally along the drawn lines. These are your "stringers".
Cut five strips of paper 6.8 cm (2 3/4 inches) long by 1.9 cm (3/4 inch) wide for the risers; and five more strips 6.8 cm (2 3/4 inches) long by 2.65 cm(1 1/16 inch) wide for the treads -- the extra 1.5 mm (1/16 inch) added to the tread width is for the "nosing" -- that portion of the tread that projects over the front of the riser.
Glue each of the riser strips to the stringers -- one end to each of the two stringers.
Glue the tread strips to the tops of the steps. The nosing should project 1.5 mm (1/16 inch).
To make sturdier steps, make the stringers out of cardboard, and use a third stringer in the center to prevent sagging.