Are you trying to cut a piece of fabric 19 x 25 from a larger piece of fabric?
If so - lay the bigger piece on a table or the floor - do you have a yard stick? Draw a line with a pencil 25 inches on one edge.
Take your 6x24 ruler and mark off 19 inches. - this mark should look like a big L.
Go to the other end of the 25 inch line and draw another 19 inch line. now you have three sides of a rectangle drawn.
Connect the ends of the 19 inch lines - now you should have a rectangle drawn on your fabric.
At this point - you could lay the large piece of fabric on your mat and rotary cut it several inches at a time - moving it as needed.
I would probably cut the 19x25 inch piece out as 19-1/2 x 25-1/2 with scissors and then tidy up the piece with the rotary cutter and ruler.