Get a think piece of paper, measure out the lithos area on it. Then cut it one inch bigger on top and bottom.
Cut the hole in the matte board slightly smaller then the lithos colored area with a matte cutter.
Tape just the top and bottom part of the paper to the back of the matte board, slide the litho in.
Also, when you cut your board, if the boarder is going to be one inch (as an example) make the bottom 1 and a fourth inch. The human eye shrinks things at the bottom when we look at them, so the picture will forever look short if you don't.
Then you can tape or glue the board to the canvas.
If you'd like I can give you about 1000 other ways to help
I'm just doing a reallllllllllly long homework project so i am just popping in and put lol. i can privde picture helsp tommorow too if you'd like.