Congrats on getting all your pieces finished!
Often sleeves say “2 the same” or something like that, can you check on your pattern, there might not be a difference between the two sleeves.
There is more likely a difference on the body front and back although not always. Do you know which is your front piece and whichnyour back piece?
Join both front and right back
This is join BOTH sleeves to the front of the jumper.
Join RIGHT BACK, means only the right hand sleeve is sewn on to the back of the jumper. The right is as you wear it (not as you look at it), I always have to do a little flip around in my thinking at this stage.
You then have left sleeve joined to front, front body, right sleeved joined to front and joined to back, back body.
It leaves the neck open at the back left so that the neck line (ribbing or edging or whatever is there) can be knitted flat. After working the neckband you will sew up that last open part, the raglan and the neck.
Does this help?
Do you need a video how to do the sewing? Mattress stitch is good.
Here’s a video