I like my pictures to be dry mounted.
-What I do is dry mount on foamcore with just a bit more than the matte size extra.
-I place the matte over the picture, carefully aligning it to the image, as I want it.
-I trace around the outside of the matte, with a pencil on the foamcore.
-Cut the foamcore to size
-Apply a bit of ATG or thin double sided tape to the top of the matte to hold it in place on the foamcore while you put the picture together in the frame.
The frame will hold the whole shebang together very well...and it looks very professional.