Here's a little circle. I used a magic loop, ch 2 for the first dc, then put 11 more dc in the loop...
Go ahead and cut the working yarn leaving a little tail, 2-3"? Gently pull on your hook making the loop so big... that you pull it out through the top of the last stitch you made.
I use a tapestry needle, so put a needle on the cut yarn you just pulled through the top of the stitch.
This is important... SKIP over the beginning ch-2 that counts as the first double crochet. In essence we are re-making the top of the first st, so if you don't SKIP the first st of the round, you will have made an ADDITIONAL st and your count will be off. So, skip the beginning stitch of the round, insert the tapestry needle from front to back pulling the cut yarn under BOTH loops of the next stitch and pull all the way through to the back.
This is what it looks like with the cut yarn all the way through.
Take the tapestry needle and thread it through the MIDDLE of the last stitch of the round. Pull gently so that the new top loops you just made are the same tension as the rest of the stitches of the round. At this point, you already have the needle threaded, so go ahead and weave in the ends. And THAT is the invisible join. :) You'll love it.