Plse! Help needed with sewing sleeves

I have been knitting for quite some time but just found the mattress stitch which has helped a lot. The video however was going a different direction and using the thread between the “v” as a guideline. I am going the other direction while sewing in the sleeves and am a little confused as to how to follow that with them going sideways

i think this is the kind of seam that you may need to do. See if the tutorial helps.