The MidCurrent site has a very good video and step by step photos on tying the Perfection Loop, a must for loop to loop connections.
Once you have tied the loop the diagram nearby will show how to make a loop to loop join that will result in a perfect ‘figure 8’ connection every time.
Learn this way of making a loop to loop connection and you will never suffer from a break-off at the loop. Take the fly-line a few centimetres behind the loop lightly held between your forefinger and thumb. Take the leader just behind the loop in the same way. Then push the fly-line loop through the leader loop until the leader loop touches your fly-line thumb and forefinger and pinch them together. With your other hand take the end of the leader and thread it through the fly-line loop and pull it all the way through until just like magic the two loops form a perfect figure-eight loop-to-loop connection.
This method avoids the possibility of the end of the leader-loop flipping over the end of the fly-line loop, and effectively acting as a guillotine on itself, when the join comes under load.