This is about joining two sides of a slip stitch work, somebody told me it's called the zipper method. Or, perhaps, there is a "zipper method" which is very similar to this.
I have copied pictures and text from my own slip stitch patterns. All three are really the same with small variations. I give you all three for better chance to understand the principle. Which loop you choose to work in, and which side of the finished row you want to use as right side, that depends on the look of the work you are joining.
You don't cut the yarn before making the joining row - or after, if you want to have the chance to try the finished item! If you find you have to add (or take away) a row or two to make it fit, you just rip the joining row, add or take away the needed rows and then make a new joining row.
This "seam" is not supposed to be invisible, instead it is meant to look like one of the other rows.
The two sides that are to be connected are always held side to side, like you see in the pictures. Yarn and left hand is held under while you crochet the sides together from above.
First example is from the pattern for a pair of socks: Hold work inside out like a tube with your left hand and yarn inside, crocheting from the outside (the wrong side). Put hook through the front loop of the starting chain, then through the back loop of the last row, yarn over, pull through all three loops on hook. The most important thing is that the two parts should be held side to side like this and not on top of each other as is normal when you crochet something together. The other important matter is to put the hook through the loop in the right direction =from the fabric towards the gap (the space that is to be closed) both times. This is how the stitches make a flat seam instead of a sharp ridge.
Second is from the hat pattern: The crochet-seam is done from the brim to the top like an odd row.
See picture 13. Hold the work like a cylinder with wrong side out. Hold the yarn in your left hand inside the cylinder. The starting chain is in the upper part and the last row is in the lower part of the picture. Here the starting chain looks like a row of stitches with front and back loops. Insert hook from above through the front loop of the starting chain, then through the back loop of the last row, yarn over and pull through all three loops on hook.
The yarn is held inside the cylinder all the time. In picture 14, you see the crochet-seam from the right side. It forms a rib almost like the others.
And last is the example from my mitten pattern:
Place your left hand and the yarn inside the mitten and the two sides next to each other like this. Put your hook through one loop of first stitch on each side, as in the picture. The direction of the hook should be from rightside/outside to wrongside/inside of the mitten. Yarn over, pull through all three loops on hook.