There are two basic categories when it comes to methods of assembly for crocheted pieces--methods that use a needle, and methods that use a crochet hook. This page will detail some of the options for sewing your pieces together using a needle. Please remember to use the same material to join as you did to crochet them! NEVER use sewing thread on a piece made of yarn.
These joinings or seams are meant for simple squares, with a straight edge, not those with fancy edgings. There are many other methods than those I've listed on these pages, like Priscilla Hewitt's flat braid assembly and the Crochet Dude's Dudessembly which look great.
Before you start, you might want to read this for some general tips and important things to remember: Assembly Basics.
Put two squares on top of each other, right sides facing. (Fig 1) I like to whipstitch in the back loops only (BLO) so I will have a pretty ridge on the right side of my squares. I also find going through BLO helps the squares to lie flat. If you do not want this ridge, however, it is okay to simply whipstitch through both loops.
Cut a long piece of yarn, about 18". Don't make your pieces too long as the constant rubbing as you sew it through puts stress on the yarn. (Don't we have enough stress in our lives? Hee Hee. Don't want to stress our yarn!) Thread through a large eye needle. I like the #16 needles. Starting at the same point on both squares (right hand corner works well), go in on the side farthest from you and come out closest to you, (Fig 2) leaving a tail of about 4". I personally like to go back through that same hole again to secure yarn. (Fig 3)
You'll continue in this same vein, enter through the back and coming out through the front (blo) all the way to the end of the row.
You may sew over the tail as you go or sew it in later if you'd rather. When you get to the end, I like to go around twice in that last stitch as well to secure it and then cut the thread about 6 " from the end. You will weave that thread in later. Note: If you don't want to sew over the ends as you go, you might bring your thread over to the left as it keeps getting caught in the hanging threads and is just a pain.
Be careful as you are sewing that you are not missing any stitches. This is especially important if you are working with squares that are made the same. You don't want them to be off. You may have to fudge a bit in between squares so you don't have holes.
Place project right sides together, thread needle with matching yarn. Secure yarn. The way I do this is to insert hook in spot where you want to join. Pull needle through both pieces, leaving a 6" tail. Reinsert hook in first piece and pass through again. Then begin the backstitch. * From front insert needle through both loops of both pieces, being careful not to split the yarn. Pull through. Skip next stitch, and insert needle from back to front through both loops on both pieces and pull through to front. Now insert needle from front to back into skipped stitch. Repeat from * until you have completed your join. If the piece has not been edged and has "rough" edges, just estimate the width of a stitch and go in and out as outlined above.
Here are some examples. They are not in successive order, but random samplings.
I pulled the edges apart to show you this. When relaxed, you can just barely see the white through the red.
This is the back that you can't see as you're working, seen from the wrong side.
This is a tricky one to explain and to understand without someone standing there to show you. It took me the longest time to figure it out, but it seems to work as described. You will find it mentioned in many knitting tutorials, but not many crochet ones. It is supposed to be virtually invisible, especially using same color yarn. I used a contrasting yarn here and you can see that it does indeed sort of disappear. There will be a small ridge on the wrong side. This method is supposed to be useful in sewing garment parts together, but I have never made a garment, except for the poncho I made for my CYCA certification so I can't verify that.
Here's how you do it.
Place project with right sides facing you, edges together (not facing one another as in other methods but lying next to one another so that you see both right sides). Thread needle with matching yarn. Secure yarn through both sides at the edge where you intend to join. With needle pointing up, place the point into the last st (you might consider this the first stitch) and take a small st, leaving yarn loose. Go to the opposite piece and pick up the corresponding row. Return to the first side, enter the last point of exit and pick up a st. (Fig A) Be sure to keep the needle pointing up to the top of the work, rather than to the side of the work.
Tighten yarn in seam after picking up 3 or 4 sts. Yarn must be tight enough to disappear but not enough to pucker at the seam. (Fig B)
See how you can barely see the yarn from the first picture once you tighten it? It disappears. This stitch also works well on edges that are not finished as these squares were that I used for the pictures.
If you need further instruction in this stitch, Knitty has excellent instructions. Although it is for a knitted piece, it would work just as well for crochet.