I've not built a tamo frame myself. I've just had a bit of fun early on finding Japanese links about how to make them. Since then many people here in the USA have made them and you can read about how they did it by reading the older post in the Tenkara Nets section of the forum, this section.
I would encourage you to open the Resources section and look at the steps taken to make a tamo. Or go to page 4 of this section and work your way back from the older post to the newer post, reading through the processes people have taken who made their own Tamo net frames.
The short list of steps are
1/ decide what diameter net you want, in the gear section are listed 24cm and 27cm nets. About 9.4 and 10.6 inches. That is - if you are going to use this type of netting.http://www.tenkarausa.com/product_info.php/products_id/113
2/ cut a branch with the right diameter for the handle, with 2 branches coming off the main branch at more or less the desired angle to the part that will become the handle.
3/ clean it up a bit and clamp the smaller branches in a circle of the correct diameter while you let the wood dry. You might want to clamp it so the circular part is flat and the handle part comes off at the angle you want. Use some kind of form to clamp it to. Metal hoop, bucket, a piece of plywood cut the the correct diameter. Let the two ends of the smaller branches overlap. However, some people like to leave circular part a bit irregular and not try to make it as perfectly circular as they can force the wood to take. Which might require steaming or wrapping the wood in a cloth and pouring hot water on it to make the wood pliable enough to persuade it into a flat circle.
4/ after it has dried sufficiently, cut the small ends off at an angle that can then be glued , pinned or lashed together to make the circular part the net frame
5/ when you think the wood has dried enough to take the finish of your choice, stain, varnish, oil, whatever. remove the frame from the drying from. Remove the bark if you left it on before apply the finish you want to use.
6/ attach the net - the TUSA video of the processhttp://www.youtube.com/watch?v=f1BLpj72XHo
Some people like to add a piece of deer antler to the handle end which is a traditional thing to do in Japan. You can read about how to do that in some of the other post here. Also read the other post to see what other people used for finishes, how long they let the wood dry , type of wood they used, etc.
Update - sorry about the broken image links. The images where there when I captured the image links. Now when I go to the web page where I found the images I wanted show it now shows the same X-icon instead of original images.
Maybe the other images from that page will be useful.http://blogs.yahoo.co.jp/urotasoniko/18430979.html