Here is an easy way to build garden wooden stairs.
It is simply based on wide planks cut that defines the structure of the stairs.
Step 1: Design
Draw you stairs and get a wide plank represented with the blue line the drawing.
Out of this wide plank your are going to keep only the blue part.
This is the base of your stairs.
Step 2: Cut the Base Planks
Buy some wide planks. Mine are 220 cm x 23 cm x 36 mm.
Draw the part to cut out of the wide base planks.
Use a jigsaw to cut them out.
Create one base for every 30 cm of stairs. (Mine are 90 cm wide so there are 3 base planks)
Step 3: Implement the Base
First assemble the bases with 3 terrace planks.
Dig dig earth to make place for the stairs.
Add some concrete and put the stairs on it.
Use a laser or spirit level to be sure it is horizontal.
Step 4: Fix the Planks
Fix the stair planks with two stainless screw at each base.
Step 5: Finish
You can go further in our garden with a longer stairs and even add some turns.