You failed to mention what size your teddy bear is. Personally, I love to crochet the miniatures. The assembly method is all the same. The only slight difference would be with the size of washer/cotter pin/floss, yarn or cording you use.
I'm assuming that you want a 5-way jointed bear? That's what I'll instruct you on, just in case.
I'll try to make it simple for you. If you need further instructions, let me know and I'll send you a URL.
1. If the head wasn't attached with a cotter pin/washer while crocheting the head, there's an alternative method.
Using a doll making needle, with thread doubled and knotted, insert the needle on the bottom end of the body, going behind one stitch and back out. Separate the thread (or whatever you're using) near the knot and insert the needle through the loop. Draw this snug and make sure the knot is tucked inside of the bears body.Continue to push the needle up through the top (neck) of the bear.
Place the head on the needle and continue to push the needle upward through the top of the head. Pull the needle all the way out, making the thread snug, but not tight. Reinsert the needle just one stitch over from point it emerged and back down through the bottom of the body. Make sure thread is adjusted to the tightness you'll like, tie a knot and hide the knot inside the body.
ARMS & LEGS
2. At this point, I'm assuming you haven't already crocheted the cotter pins inside the arms and legs. That's okay too. A cute way to do this is to use 2-holed tiny buttons (they don't even have to match). Start by using the double threaded needle with the knot and insert it directly into the bottom side of the button, pulling the needle and thread almost all the way through the button and back down the other hole. Catch the loop next to the knot as you go.
Place the arms in the position you want them to be in. Now, run your needle through the one arm (in the area that would be the top part of the human arm-before the shoulder and actually a bit lower), through the body and out the other side into the other arm (don't worry if you don't match up where you want it, just withdraw the needle a little and reinsert). Place the button on the needle and continue to draw the needle all the way through. For larger bears it's probably going to be easier to do this will the bear is laying flat on your work surface.
Push the arms in place and draw the thread snug (but not tight) and insert the needle in the other button hole, through the body, out the other arm and button hole (yes, I go through the first button twice). Insert needle back through button and pulling out between the button and but body limb. Stitch a knot in place and tuck it into the body to hide it.
Use the same method as for the arms; making sure to try several areas to pin the legs before starting.
Keep in mind, with 5-way jointed bears, the arms, legs and head will turn all the way around. But, if little hands keep turning them the same way without turning them back the opposite way, the thread can get tight and cause weakening and possible breaking.