Help pllease

[Acidburn]

I need help under standing this joining...

[Granny Square]

Hello again!  

You need to start a foundation DC row with a short chain, right?  And then the foundation stitches kick in with that extra loop at the bottom that you work into.  So in this tutorial, looking at the photo where it says "That's it! First foundation DC made", you can see the initial chain, and then the first FDC.

So joining in the round like you'd need for a sock cuff, you are joining the last FDC to the beginning of the round, which is 2 chains in the linked tutorial.--so joining in the topmost of those 2 chains (or 3, depending on YOUR pattern's instruction to start the FDC round) .  I hope that makes sense?  Really, the same as if you were joining 'normal' DCs in the round, where there are usually 3 chains that take the place of the very first DC, you'd join in the topmost of those 3 chains.

[Granny Square]

Just throwing this out there, if you are struggling with the foundation chain--foundation stitches ARE stretchier than starting with a chain, then turning and DCing across.  However, foundation stitches are never necessary.  I crocheted for decades before I'd ever heard of them.  For a sock cuff edge that I wanted to make sure was plenty stretchy, I'd probably just make a foundation chain with a size or 2 bigger hook, then switch to the right size hook for the rest of the sock.