I will be installing wrought iron spindles under the railings with the retro kit on the staircase. What is the best way to transfer your line from below to above? I have tried using a level but it doesn’t seem to work out very well with transferring the line because you can’t hold the level exactly on your line below and then leaning against the railing above at the right spot and then transfer your line to the underside of the railing to drill the three-quarter inch hole. Does anyone have any tricks for laying out the holes In the bottom S and also in the underside of the railing so it lines up?

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Laser

Mark your first two baluster centers on the stair case and set a pair of dividers to that distance
Use a small plumb bob to transfer the first center to the underside of the rail.
Starting at the first center mark on the underside of the rail, mark the rest of the centers with the dividers
Mark the rest of the stair case with the dividers

Laser Level no need for any other bs

New school laser, old school plumb bob

Thx everyone. I draw in AutoCAD. I’m able to determine the spacing in between each of the spindles, but without Acad, I don’t know how to transfer that spacing dimension to the bottom cap. I have seen some videos of people using a compass to mark from one to the next, but without AutoCAD, I’m not sure how to determine measuring down the length of the angle the cap, the center of one spindle to the center of the next. Any advice?

Buy a laser. Swivel style

I like our Dewalt DW088CG

Generally, I dislike Dewalt tools. But, that laser gets really good reviews. You can't go wrong with any of the decent brands like Bosch.

Lasers are one of those why didn't I buy one of those 20 years ago? Maybe 30 now.

You can also clamp a stick to a smaller level(old school but not as cool). Plumb bob would drive me nutz.

Only need to use the plumb bob once, they don't need batteries and they don't cost $145.... by the time you set up a lazer to make the one mark, plumb bob would be done
To the OP, AutoCad will give you the dimension along the pitch of the stairs, you want the aligned dimension between the baluster centers
Set a pair of dividers to that dimension and step the centers off
Or, do a full scale layout of three or so balusters and measure the distance between centers that way

Place your previously cut to length rails on the curb below. use a combination square or similar and transfer a mark on the curb up to the rail sitting above. Repeat as needed.

Above is the Old guy solution. The New guy solution is the laser level. But I would still keep the hand rail right low.

I can set my laser up on a step or nearby faster than i can unwind the tangled string on a plumb bob.

I made this video years back, but the same theory applies.

http://https://www.youtube.com/watch?v=CDnAc_smxfE&t=4s