For the last 3 years, I've collected some leftover/throwaway carpet (and pad) and had The Incredible Hulk stand guard over it. Ironically, my neighbor DaveM just re-did their living room, so I got some giant pieces of carpet and pad and ended up with way more than I needed. The first weekend of May was cold and rainy/snowy, so I was able to put about 10 hours into this and it goes fairly quickly. I tossed a bunch of the carpet and pad up into the crawl space. The pad is easily trimmed with scissors and a staple gun is used to anchor it - just want to make sure it doesn't move as I overlay the basement carpet. The monster piece of carpet measures 14' by 18' - this was a bear to move and get positioned, but it sure filled up a big chunk of the crawlspace and reduced the amount of fill-in required. I used a box cutter knife (cut from the back side) to trim the carpet. I decided not to use a knee kicker or power puller to stretch it but instead pounded in some 1/2" by 1" "nails" to anchor the carpet and then pulled it by hand, hammering in nails every 5' or so. Seems to work just fine with no bunching. It was kinda like a giant puzzle after that as I would lay out the remaining pieces to see which fit best and then filled in the gaps. I trimmed the touching pieces as best as possible and used duct tape on the bottom and those nails to make 'em match up semi-decently ... you may want to use a glue gun for the no-seam look.
Next up: start moving my basement junk into the now carpetted crawl space!