For Dacron I use a Bimini twist to make a long loop. I then fold the loop over and tie a two times through surgeons knot (often referred to as a double surgeons). That leaves me with a single line to the Bimini, two strands to the surgeons, then two loops. You want the final double loop big enough to pass a reel through. The two loops are looped to the end of the flyline. If you have trouble with the Bimini you can use a 6 turn surgeons knot to make the first loop with all else the same after that. This connection will be very strong and allow easy line changes. I actually use this connection on every rig I use from 1wt to 14wt.

Alternatively you can just use an Albright knot to connect backing to line. Not quite as secure but good enough for most situations however you need to retie if you change lines