This is pretty simple, but worth explaining.
Your bell siphon drain pipe needs to go through a hole at the bottom of the grow bed. Naturally, you want the pipe to drain only when you want it to and need to make sure the seal is otherwise tight.
Cut a hole at the bottom of the grow bed where you want the drain to go. Use a hole-saw that is just slightly larger than the male adaptor you want to connect to the end of the 1" drain pipe.
Ensure the male adaptor goes through the hole, and screw it to a female adaptor on the other side. You may need to buy some rubber washers to ensure there is no space between the male adaptor and the grow bed floor. This is especially critical in a barrel system, which has a curved floor.
In order to get the male and female adaptors to fit well together, you may need to cut the male adaptor down so that the amount of thread on the other side is just enough to tighten the female adaptor down tight.
Cut and uncut male adaptor with and without washer.
Test your siphon and make sure it works!