The collage tool in PicMonkey is accessible from the left tool tabs bar. You can also start with a collage from the Create new button on the homepage.
A collage can be any size you like, so choose a pre-set size for social or print, for instance, first and then select a collage layout. Choose from one to 64 cells. You can always add or subtract cells later in your design process. Add photos and videos to collage cells, or set the background color of the cells. Add text and/or graphics to your collage as layers on top.
There's a lot to know about getting the most out of collage, so check out our numerous articles for tutorials and inspiration.
Collage Tool (help article)