Before starting, you should know that you'll be sewing as if you were crocheting in a" back loop only" and "front loop only "style. If you're new to crochet, that means you will only pick up half the stitch with your yarn needle (usually you'd do this with your hook). Hopefully, this will be clear following my photos. Why sew, instead of crochet? Because, even if you use the flat braid method, you'll still get a ridge. If you follow these steps, you should not be able to see the seam.
If you are planning on blocking, it's better to block the individual sides before sewing them together. You should use a grid or measuring tape when doing this, because you don't want one side to be blocked bigger than the other! I didn't block my top, because it's cotton and I know I won't have the patience to re-block every time I wash it, since I plan on wearing it often. I always leave long tails when crocheting and sewing (if I haven't made a knot). This is useful for putting the right or wrong sides together as well as for making knots. I should tell you, that I usually weave in the ends and do not knot when I crochet, but when making a garment, espescially one that will be thrown in the washing machine, it is absolutely necessary to make knots. You wouldn't want hours of work to come apart at the seems would you? So, when I'm finished, I either make a surgeon's knot, or a triple knot and pull as tight as possible, then cut as close as possible to the knot. If you've made the knot correctly, this will not pull your work. If you have a little bit of yarn sticking out, you can put a little clear nailpolish on it to prevent from fraying, but you should cut as close as possible, like I said. I have used yellow yarn for better visibility but, obviously, you'll use the same yarn you used for your garment.
One last note: If you're sewing a seam for a very lightweight garment, you can choose to do only the "back loop only" part and skip the front. When you do both sides, the seam is heavier, but has a stronger hold, because you've worked it twice. If you have a lace weight garment, doing only the back side is better because the seam will stay light and is absolutesly invisible. I do recommend doing both sides in the shoulder seams, because there is more stress in that area when the garment is worn, but you may not even need it there if it's a delicate one. I have worked both sides on my top.
The photo tutorial shows how to do the side seams, but after lining up your work, you should start at the shoulders, using the same technique.
Here we go.......
Now measure your yarn and cut. Two and a half lengths of your seam is usually enough ( one and a half if you're only working one side). Thread you needle, do not make any knots.
Turn your work over to the right side, run the thread through as shown below, picking up the front loop only (which is still the closest one to you)
Now, sew through as before, working through every front loop only, until you get back to the bottom, turn your work again and bring the neddle to the back and up a stitch. Now bring the tail up a stitch and make a secure knot using the two ends. (This is not shown) Pull as tight as possible and cut as close as possible.
Now you're ready to sew the other side seam.
I hope this tutorial has helped you. As always if you have any questions, need help, or have found an error, let me know. I'll be glad to help! :)