Your Dotstar strand should have connectors already soldered on to both the "in" and "out end. The "in" end usually has a female connector and the "out" has a male. But not always! Use the arrows on the strip for reference, find the "out" end and cut that connector off, setting it aside for now. Later you'll solder this connector to the Pro Trinket, so you can plug it into the "in" end of your strip.
Cut your strip to length starting at the "in" end…measure 20 LEDs down the strip, then cut carefully through the silicone sheath and then cut the strip betwen solder pads. For a small to medium sized fire, 20 lights is a good length -- too many more and the battery will quickly drain.
Look at your strand before cutting! Use the arrows for reference…they point from “in” to “out.” The factory producing Dotstar strips has occasionally made changes between batches, so you shouldn’t assume a certain gender to the end connectors.
To seal the end of the strip and keep moisture and ants out, fill the cut end of the silicone sleeve with some hot glue. You can use a piece of heat shrink to hold it all neatly together.