Snip snip…I have cutting questions. Sewing Machines Parts and Sales in Los Angeles

I need to cut a 32 in. and 42 in. square of flannel.
I have a rotary cutter, 24 x 18 mat, and 6 x 24 ruler.

I have looked on Pinterest and youtube for tutorials or suggestions on how to best accomplish this, but I keep finding stuff for cutting small squares.
I feel like it’s something obvious I’m over looking. Any suggestions or guidance is much appreciated.