Both parts must be v carved for this to work. They look like they won't fit but they do (at least so far). It can be difficult to get the parts to fit because there is no indication that you are close until they suddenly fit. I always fit mine dry (no glue) and make pencil guide marks. That allows me to quickly find the fit point when I have the parts wet with glue.
If done correctly, the male and female parts will contact each other only on the v carved sides. Heavy clamping pressure wedges the two parts together. (One user reports that he uses a hammer!
Using lots of glue helps to fill the void in the bottom of the female part. Give the glue plenty of time to set before cutting the backing off the male part. If you use conventional wood glue, wait at least 24 hours. The waiting is the most difficult part of this process!
It seems as though a 60 degree v bit is being used more often than a 90 degree bit.