Step 7: Solder the solar cell
Just lay the solar cell down and align the wires so that they touch the cell's solder points. It does not matter which wires goes to the positive point and which goes to the negative point.
You'll also want to determine how far you want the motor to stick out of your roach. I like keeping my motor under the solar cell, so they're not sticking out the rear, which means that I don't use the full length of the wire.
Place your soldering iron tip so it is touching both the wire and the solder point, while keeping your solder on the other side. You don't need to go overboard with solder, just bit will do.
Repeat this on the other side.
Once dry, turn your solar cell over just to make sure everything is sticking. If you'd like you can go outside and test it out. Under bright sun your motor should turn.