For Graham Harrison and others. I'm sure others will add their tips.
a) always shape the flex track before fastening it down. Do no start fixing it at one end and curving as you go.
b) make a track template of the design radius, a quarter circle is long enough. Also make a transition template: make the true curve about 1/8 of a circle, and add a transition plus about 1 foot of straight at one end. Use the templates to draw the location of the flex track on your baseboard. If you make it of 6mm ply, you can use it as a jig to help shape the track. The width of the template depends on your track base. If you are using a ballast strip, make the template that width. If you are laying the track directly on the baseboard, make the template as wide as the sleepers. It will work for both left and right hand curves, just flip it over. If you make several templates for different radii, mark each one on both sides.
c) if a joint will fall on a curve, use railjoiners (fishplates) and solder it before curving.
d) overbend the flex track when curving it. You want the flex track to lie without straining to straighten out, especially where you need an electrical gap. Curve it enough so that it will spring back to the curve you want, instead of being held in place by the little bits of plastic that stand in for rail fastenings.
e) when the flex track lies on the curve without straightening out, offer up the adjoining piece, and mark the rails of one or both pieces with a marker for cutting. Use a side cutter to cut the rail from the top down, and rotating the cutter downwards slightly as you cut. This will give near perfect vertical cut. The flat side of the cutter goes against the good side of the cut. Dress the rail end with a small file. NB: Wear eye protection, as the bit that's cut off can fly off at an alarming velocity.
f) flex track takes a natural easement (transition), so make the pieces long enough that they are straight for a foot or so before the nominal start of curve.
g) gaps on curves are a problem, it's best to avoid them, but that's not always possible. Some people use 3 or more PC sleepers either side of the gap to hold the track firmly in place, and stagger the gaps. Others (me too) overbend the flex track so that the rail is actually curved, and not merely held to the curve by the sleepers. Cut the gap after the track is fastened down.
h) solder the feed wire to the bottom of the rail, and drill a hole in the trackbed to accommodate it. Neater than the wire soldered against the outside of the rail. It's also easier to solder on the workbench.
i) keep all sleepers removed to accommodate soldering, rail joiners, etc. Cut the fastenings off, and you can slide them under the rails to fill in gaps in the sleeper strip.
j) a pair of pliers will do as a heatsink.
k) sight along the track at a low angle to check for straightness and smooth curves. Adjust everything _before_ pinning and gluing.
Safety note: use only water-based latex acrylic glues for gluing ballast strip, ballast, and track. Solvent glues are not only very bad for your liver and brain, they also tend to warp the plastic sleeper strip.
HTH