Good tarps are a necessity in the winter time. They do everything from keeping firewood dry to aiding in holiday decorating. Sometimes you can get a good deal on smaller tarps, which you can then fit together into a custom size. To do this, you can use a good tarp tape, tarp glue (which works only on vinyl tarps, not the more common polyethylene ones), or you can use a soldering gun or plastic welder to "glue" two tarps together by fusing the plastic they're made of.
Lay out the tarps as you want to glue them, and line up the seams to match each other neatly. You may want to tack the tarps in place using some duct tape or other tape.
Apply the specially formulated plastic glue to the two tarps to be attached as directed in the packaging. Lay down a line of glue in lengths short enough for you to manage easily, while keeping the tarp straight and even. Don't expose the glue to air long enough for it to begin drying.
Smooth down the seam and make sure there are no gaps in the seam where the tarps' waterproofing could be compromised. If there are gaps, go back and put more glue in those areas.
Lay one tarp down flat on a large work surface. Position the other tarp as desired, but folded back onto itself at the area where the seam is to be.
Working with manageable-sized chunks of seam (about 6 inches or so), use the welder or soldering iron and direct it at the plastic welding rod, leaving a small bead of plastic near the fold on the tarp that is laid out flat.
Unfold the second tarp to overlap the first quickly, while the bead is hot. Using your gloved hand, press the two layers of tarp together. This should result in a watertight seam.