The are 3 you can make stairs:
1) Create using stair tools
2) Create from 2 linedefs feature
3) Do it all by hand
1. Create Stairs using stair tool
Stair tool mode is entered by quickly double right clicking the mouse and selecting the stair tool or by selecting the Object menu and selecting stairs.
Stairs are always created inside a Sector. The bottom step connects to this Sector for reference. You need to complete the top step by connecting it the Sector desired.
Switch to Vertex mode and place the last 2 vertexes on the LineDef desired and reply yes to the split LineDef message.
You can also do this by dragging the stair on top of the LineDef you want it to connect to.
Finish by changing the LineDef just created by the split to point to the Sector of the last step ( Sector reference ).
Note :Whenever you make more than 16 steps, be on the lookout for HOM errors. You are creating a large number of 2-sided lines with varying Sector heights. It depends on the surroundings.
3. Make Stair from 2 LineDefs
Select 2 LineDefs that will be come the Stair sides and then activate this function from the Object menu. The LineDefs, Ceiling height, textures, etc., are modified accordingly.
The word BOTTOM indicates the bottom of the stair default. Reply Y to the next prompt to make the bottom the TOP!
Enter a negative value to make the stairs go down. This is similar to reversing the stair direction, but the steps are created for going Down instead of Up.
Check that the stair did not overlay some existing area. Sometimes you need to fix the final sector, if the steps end up overlaying an existing LineDef.
The TOP of the stair changes the floor height of the old Sector! So anything that that connects to the old sector should be raised to match the new floor. Make the stairs first to avoid extra work.