The two responders have stated that you should lay the flex lengths as tangent track, join them, and then solder them. Afterward you can flex them. That is correct.
But there's a bit more: First, you really can permanently bend flex track. Takes some doing, some courage, some practice....but you can wrap it around curved blocks of wood screwed to a working surface, held down so that it won't flip up on edge, and bend it permanently. That helps.
Secondly, the ends won't bend....period. So a trick is to slide the sliding rail about four or five ties deep into the other length of flex, necessarily displacing that section's sliding rail by the same amount. Use a half-joiner there and solder it. The spike heads on all those ties will help to bend the rail ends and it will make the non-joined non-sliding joint further back do 'better'.
Otherwise, with strong fingers, flex and bend the rail length ends yourself. It can be done, as can anything we put our minds to. But a bit of curvature will really make a difference.
Lastly, and this applies to every rail end, no matter if it is on a turnout or on lengths of track, for ends the factory cuts or that you make with rail nippers....file them. Take a small metal file and champher/bevel the tire running tops of the rails and also the flange faces. Attached is a diagramme showing what I mean.
This is very important. It is even more important when your curves are tight and you have joints partway through them. Flanges in tight curves press against the flange faces on the outer rails, and they will find chinks and rough edges, or mis-aligned rail ends. So file 'em!! By doing this, you create mini-ramps or cams that the flanges scraping the flange faces will find much less obstructive.
-Crandell
