I have learnt quite a bit from my the last currach/coracle/bullboat I made. On that one I did a lot of mistakes, most of which I managed to avoid this time. This site also gave me a lot of needed new inspiration.
First of all, what you need to get is a cow hide (or some other large animal). Making the frame takes about 1 day, but dehairing the hide in a stream takes substantially longer.
The frame is started by sticking a circle or semicircle of an equal number of rowan rods into the ground. They don't need to sit deep, but they should preferably be stuck to an angle outwards.
Next you start a special type of weaving where you bring in one and one rod so you get a type of diagonally overlapping weave. The currach site explains it better.
The secondary weaving is done in the ordinary fasion with one round at the time before starting an opposite one.
Bend the withies down and pull 2 and 2 over eachother. This will give the boat a slightly more rectangular or square appearance in the end. Tie the crosspoints with strong cordage or leather. There was in my case no need for it to be weighed down for the recommended 10 days.
Take the dehaired hide and stretch it gently on. Use the longer parts, the legs and such for rope and tie it to the frame. I didn't bother to cut the protruding points and they made it easier to tie the hide to the frame.
The currach ended up quite big and very stable. Here is a photo of two of the viking voluntaries, Brage (14) and Gaute (12), rowing the currach with oars.