I did my TR250 top, first convertible top I have done, didn't find it that hard, I had a frame but no old top to use as reference.
Basically find center by folding it in half, mark with chalk (sidewalk chalk works great, if you have kids or know somebody that does they will have some). Fit the rear snaps to the rear rail center first, work your way forward, stretching so it is taught and wrinkle free, but not too much. Header rail fits with contact cement, then is also held on with riveted on bar. I did find that you can pull it up off the rail and do it again if the fit isn't quite right.
Mark the various snap points with sidewalk chalk as you stretch it over them, then you will know precisely where to make the hole.
More detailed instructions here:
https://www.vtr.org/maintain/tr6-top.shtml
and here
https://www.buckeyetriumphs.org/technical/TR6Top/TR6SoftTop.htm
I got very good results, no problems whatsoever, biggest issue was header rail gluing, I did it a couple times before I got it right, but easy enough to pull off and start over. It is a do it once and do it right kind of job though, once you make a hole in the top that is your spot for the snap, but the chalk method makes it pretty goof proof, "measure thrice, cut once"
Do it on a warm and sunny day, as alread mentioned.