Joining squares for a beginner

Help! :doh: I have only knitted squares (for a collaborative project that I didn’t put together) and scarves and decided to make a baby blanket for a dear friend. I liked the idea of the different colored squares put together, but I have now realized that I have NO IDEA how to join the completed (casted off) squares together. :?? My yarn is a bit fuzzy, without much give. Would a mattress stitch work the best/easiest? Any help is MUCH appreciated. :XX:

Mattress will look nice on the front, but will leave a seam on the back.

The edge-to-edge seam might be what you’re looking for.

Thank you so much! That looks like it will work!