If this is your first heat-sealing project, I recommend trying out your iron on a few fabric scraps before starting on your packraft (scraps are included with every kit). That way, if your iron is too hot you won’t melt a hole in your packraft fabric, or if it’s not hot enough you won’t waste time making inadequate bonds.
The Proper Heat-Sealing Technique page describes the heat-sealing technique in detail and I recommend you read it so you can be sure your iron is the right temperature and you are making good strong welds. The information on that page will also help you troubleshoot any problems you run into while heat sealing.
Note that in my instructions I use the words “join,” “seal,” “weld,” “bond,” and “heat seal” interchangeably. The meaning is always to apply heat to the fabric with your iron so that the TPU surfaces melt and fuse together.