Finally I sewed them onto the quilt. After finishing, one part I curiously took a break and check other tutorials on the topic. These other tutorials ask for a wider strip and instead of sewing it like I did (one edge first at 1/4" seam). They sewed it at 1/4" seam while folded.
Did I do it wrong? I'll keep going because although it might take longer, I think I'll get the same result.