ELP! quick! I need to connect sleeves to infant cardigan!

m doing the lion brand seed stitch cardigan, and I need to connect my sleeves to the body, but I already mattress stitched the sleeves shut, what stitch to I use to connect the sleeves to my body? Do I need to use the matteress stitch to close the sides of the sweater first?

I answered your PM, but here’s the link again for anyone else who can use it.