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Seam
Whichever method used to weld pieces of polyethylene, do not try and bond them edge-to-edge. Although you may get a successful weld it will never have the same strength to resist tearing as rest of the plastic. Instead, do a simple seam fold to first bring the pieces together: lay about a quarter inch of material one sheet over the other and fold the two pieces over together. This will not only make a strong seam but when welded it will also be water and gas tight.
Heat
The trick to using heat to bond pieces of polyethylene is using the right amount of heat from the correct source. You want to bond the plastic together, not melt it and damage it beyond repair. Also, applying direct heat can lead to the plastic becoming sticky and adhering to the heat source and again being damaged.
Lay the polyethylene seam on a flat surface and cover it with a thin cloth. Then use a dry hot iron and pass it repeatedly over the cloth, raising it and the plastic's temperature. In just a few passes the heat will melt the seam and fuse into a single weld.
Adhesive
Some adhesives are too caustic for polyethylene. Model airplane glue with its acetone base will melt the plastic before welding it. Adhesive tape will take some doing to apply but will work to weld the seam. Just remember when using tape to apply it to both sides of the seam for a strong weld.
Fasteners
You can sew the polyethylene seam to weld it just like pieces of fabric. The real technique to this procedure is using a sewing machine with a wide offset stitch pattern. Another way to weld the pieces is using continuous staples. If the polyethylene material is thick and heavy you can use lightweight pop rivets spaced along the seam. The only drawback to using fasteners is it will be a strong weld but will not be very water or gas tight. One solution is to use the fasteners to secure the pieces together then apply heat to seal the weld tight.