Seaming garter stitch

hello all! i just finished my first hat! it has some squares that are in stockinade that i can use mattress stitch to join, but the rest of it is in garter stitch. what’s the best/most invisible way to join it? thanks for the help!

Garter stitch never seams as invisibly as stockinette in my experience, but you can make it look nice. Scroll down this link.

Here is another site that shows seaming garter stitch. LINK