This is quite a difficult task, to say the least.
Lay the template down on the stair treads, up against the wall on the angle it will be installed in, with the corner of the end of the board just resting on the lower floor. Fasten this board to the wall or have someone hold it steady.
Lay a framing square on top of each tread with the tongue up and the body down on the stair tread. Now push the square so the tongue touches the nosing and is up against your template. Scribe the bottom of the square out along its length and along the tongue on the nosing side up above the stair tread. Start at the bottom step working your way up the stair stringer. You will see that the tread and nosing lines are intersecting.
Now remove this board and lay it down on the flat. Measure how far the nosing sticks out past the stair riser. Scribe a line inside the nosing line, on the board, by this much. Measure how thick the stair tread is at the nosing and scribe a line on your board accordingly. Do this for every stair step.
Cut out below these lines to match the contour of the steps. Now place this template in position and see how it fits. This is where a set of scribers will help. If you don't have scribers use a compass. Fasten the template on the wall, again, and set the scribers to the largest gap showing and scribe around the horizontal lines if needed or along the vertical lines if needed or both. Remember that the template needs to move the way you scribe it so allow for that. Hopefully the amount of the scribe needed will be maybe 1/4" at the most and this 'movement' will not matter.
When the template fits okay, notice the lower end where it meets the floor. At this point the board should go along the floor then up 90 degrees to meet the top slope (the top slope should not come down to a point on the floor). Let the top end run wild—don't cut it off—until later.
Once you are happy with the fit of the template on the stair steps, lay it carefully on the finish stair stringer and scribe it. Cut this out carefully. Lay it in position and carefully scribe it to remove any deficiencies. When you are happy with the fit, cut off the top end where you want it.
Like the bottom end the top should meet the top floor at a 90, as well. Now that you have installed the stair stringer to the wall, you have some options: If the steps are painted and the stringer is painted, to fill in any imperfections in the fit, just apply some paintable latex cauking around the contours of the steps against the stair stringer.
If you can't possibly get the fit any better, but you are not happy with the fit, and the stair treads are carpet or hardwood and the stringer is hardwood, you could cover the gap with a small molding or a caulking to match the hardwood (if step and stringer are both hardwood).
Dave(Ask Dave) (About Dave)
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