Laying and joining flextrack on curves is something that can be daunting and frustrating for beginners as well as experienced model railroaders alike, but here is the technique that has served me well over the years. The following pic shows a piece of Peco 55 flextrack in place on a 17" radius curve on my helix, waiting for the next piece. First things first is always put the movable rail on the inside of the track.
When putting the movable rail on the inside that inside rail will end up being longer.
Using a cutting tool (in my case a Dremel with flexshaft and cutoff wheel) I then trim the inside rail to match the length of the outside rail. You can see the red mark of where to cut the rail.
After the first piece is trimmed to length I lay the second piece out, movable rail on the inside, with a radius close to the final one.
Now the second piece needs trimming on the inside rail.
Once the second piece is trimmed, attach the two rails together with rail joiners.
Next solder the 2 pieces of flex to each other and the joiners.
Here is the soldered rail.
A little cleaning up along the top edge and inside edge with a needle file to remove any stray burrs, giving a perfectly smooth joint.
Now pull the track out of the way towards the inside and put down the adhesive latex caulk to hold the new piece of track.
Smooth out the caulk and set the new flextrack piece in place, tapping in some thumbtacks to help hold it. The caulk will turn clear when dry.
All done, or as in my case: Lather, Rinse, Repeat!!!