Well, you have been knitting around and around over 72 stitches, 24 on each of 3 needles. Now at the beginning of a round you bound off 1/2 of those, all the stitches on needle #1, and half the sts on needle #2, then you worked on across the rest of #2 and all of needle #3. What you are going to be doing is to cast on new stitches to replace the 36 you cast off and you want them to end up right over top of the ones you cast off. You are now at the end of needle 3. Proceed as follows:
Put needle #3 in your left hand so that the working yarn is coming from what is now the first stitch of that needle. Cast on 36 sts to needle #3 using either a knitted on cast on or a cable cast on (I usually prefer a cable cast on, but that is personal preference) (there is a video of how to do these 2 cast ons on this site in the "Increase" section. Here is a LINK. You have 2 free needles at this time, use one of those as the working needle and either knit into the first st at the tip of needle #3, or between the 1st and 2nd sts and pull up a st loop and put it on the needle in your left hand. Repeat that until you have added back 36 new stitch loops.
Now move the 36 sts to one of the free needles. Hold the needle with the 36 new stitch loops in your right hand and in a way that they are spanning across right over where you cast off the 36 stitches before. Have the cast on edge facing down and the new stitch loops on top of the needle. There is a needle to the left that is only holding 12 sts, take that needle in your left hand. Now take a free needle in your right hand and join the work into a circle again by knitting the first stitch in your left hand, using the free needle. After you knit the first st pull it up close to the needle that was holding the stitches. And after the first stitch you can place a st marker if you'd like, the round beginning will shift to this place (or you can join it there but leave the beginning where it was before if you'd like. I don't think it will mess anything up either way you decide to do it). Rib around the 36 stitches that were not cast off, then continue to rib around the 36 new stitch loops. You now have 72 sts again. You have just completed the first of the 5 rounds they ask you to do. If you'd like you can redistribute the sts so that there are 24 on each needle (take into consideration what you choose to designate as the beginning of the round).
Then you dec back to 64 sts and finish the sock. I managed to find your pattern on the first try. I was lucky. If others are interested this pattern is a free download at Vogue Knitting. Here is a link, but you have to be a member or join (free) to see the pattern. LINK
There may be other ways to accomplish the join, but that is what I did on my sample "sock" and it will work well.