We are total novices, so if (when!) I get the terminology wrong you aren't allowed to laugh at me..
We put some hold-on eyelet groundsheet clip things on the corners of the carpet and used a bit of string to attach them to the bedroom toggle eyelet hole thing, one on each corner.
love the 6 inch nail idea but sadly husband does not agree, so thanks have now got a idea of how going to go about keeping carpet in place, (may just forget to pack the thing in the car, sure hubby would never notice).
I just got a carpet of ebay, quite pleased for £45, just need to sneak it into the garage, not sure how I am going to explain that the picnic rugs have grown over winter into a wall to wall carpet !