Don't cut up your playmats just because the table size changes in 9th edition 40k.
To start with, the 9th edition offers new table sizes to use depending on your games. These are actually multiples of a single Kill Team board (22x30) because that's the size of their packaging (boxes), it's that stupid.
We've actually heard people say they'll cut their rugs to fit. The thing about cutting something is you can always take more off, but you can't put it back on!

Not to mention that GW must have thought that there was a hemorrhage of money taking place in the game mat industry. So, just to really annoy us, they changed the mat formats and even the objective formats!

That said, these new sizes are only minimum measurements. They are optional but can still be played on a board of the size you are used to.

Solution to the false good idea!
While we wait to see what the competition will do (apparently everyone has moved to smaller formats and/or is keeping the old formats), an easy solution for the new sizes is to use a roll of 5cm black Gaffer tape!

Yes, masking tape is a great quick fix that we found on Spikeybits, who found it on Squad Marks' Instagram...
BUT, the GAFFER is so strong, that I'm afraid that the day you take it off, the fabric will tear off with it!
For a quick and temporary solution, you can take some low-strength tape and cover the measurements you won't need.
So the solution is masking tape that you paint black.
The reason you should use a light-duty tape is that it's easy to remove and won't pull any of the carpet with it when you peel it off. You should probably avoid tapes like duct tape and electrical tape, just to be on the safe side.
So, you know what NOT to do , so don't cut your carpet and no GAFFER tape ! But if you do it anyway, optimize it!