Clean the plastic with soap and a degreaser, if necessary. Sand down the edges to be welded just enough that they are not jagged.
Plug the soldering iron in and allow it to heat up for a few minutes.
Push the two pieces of plastic together. Run the soldering iron tip back and forth over the seam to partially melt the plastic together. Adjust the pieces of plastic to get them in the proper places while the material is hot and soft.
Add more plastic onto the seam to strengthen it by laying smaller pieces of plastic along the seam. Melt them fully by pressing on the small pieces with the soldering tip until they become liquefied. Spread the melted plastic evenly along the seam with the soldering tip.
Go over the seam and surrounding area with the soldering iron using fast, smooth strokes to create an even, smoothly welded surface.