Perform a test weld using scrap material; allow your weld to cool fully and perform a rip test by attempting to tear the two pieces of material apart. If you can see the scrim of the material, the weld is good. Further reading:
Ensure material surfaces are clean and free from contaminants such as moisture and dirt.
Secure your materials to a flat, stable surface with tape, making sure they are flat and taut; ensure any graphics are properly aligned. The material overlap should be approximately the width of your nozzle.
Insert nozzle at an angle between the overlapped material
Place pressure roller on top piece of material, close to nozzle and apply pressure in a back-and-forth rolling motion
Move nozzle and roller simultaneously along the overlap until weld is complete
Allow the weld to cool complete.
Run a seam probe or flat-head screwdriver along the edge of the weld to test for gaps or voids, re-welding areas if necessary.