Don't just sew the squares together. Crochet them using a "Flat Braid Join" or "Lattice Join" in a contrasting color (like navy or deep teal). This creates the illusion of leaded glass or grout.
Most people are referring to the famous "Persian Tiles" pattern by Janie Crow (which is a gorgeous, paid UK pattern). But what if your yarn budget is tight, or you just want to test the waters before committing to a 100+ hour project? crochet persian tiles free pattern
*Ch 3 (counts as dc), work (2 dc, ch 2, 2 dc) in next ch-1 sp (corner made), dc in next sp. Repeat from * three more times, working the last sequence into the starting corner space. Sl st to join. You should see a square forming. Fasten off Color A. Don't just sew the squares together