You should just buy the clips from Reptile basics, or just get a strip of Flexwatt pre-wired from them.
If you want to solder your connections this is what you need to do.
First scrape off all of the plastic over the metal contact where you ant to attach the wire.
Next poke a hole through the metal strip just big enough to get the wire through.
Next is the important part. get some Rosin Soldering Flux and some silver-bearing solder (62/36/2).
coat all metal surfaces (wire and contact) with a thin layer of the flux.
Heat up your soldering iron to 700-800 degrees F.
Coat the tip of your iron with some solder ( just enough to make it wet and shiny not overflowing)
Place your wire into the hole and get ready to solder it into place.
now touch the tip of your iron to the metal contact strip on the flexwatt and the wire at the same time. this will heat up the contact so you need to do this step and the next step as quickly as possible.
after a second or two touch the tip of the solder strand to the contact strip and the iron tip at the same time. allow the hole and all of the wire to be coated with the solder, then remove the solder an iron and let it all firm up.