Carpets are tedious, they have to be trimmed to fit, take your time. The rear seat cushion lifts out, it has a catch on each side and the backrest is bolted in behind the seat cushion, then lift it straight up. The rear carpet need only be trimmed at the sides, most important thing with the front is to line up the toe pad correctly and cut the hole for the dimmer switch in the right place. You will also have to remove the sill plate trim. I had my 4 year old daughter sit on the hump to keep the carpet in place while I trimmed it. Also best if you take it out of the box and hang it for a while so it straightens out.
As Dan said, the bottom cushion has a metal catch underneath with a back facing notch. You have to sit on the cushion or push down on it very hard and then at the same time, push the cushion toward the back of the car, that should release the catch. Same procedure to re-install it. You can see the catch on each side in my pic here.
Click..is Jim..former owner of a 69RS-LM1 350/255 - 700r
"There will come a time when you believe everything is finished,
That will be the beginning." Louis L'Amour
an important thing to remember cutting the carpet, take your time and cut very little at a time. always cut off less then you think you need to and kept trying the fit. you have to be very careful not to cut the carpet too short on both side edges.