It started with this idea, but I adapted it and did it my own way.
around a yard of fabric, (or half yard of each of two fabrics), or lots of scraps.
velcro or snaps for closures
interfacing (optional, but better with it)
cutting a straight line
sewing a straight line
1--Choose your fabrics. I used lots of leftover scraps, so I just divided them into 'reds' and 'greens' however you can do something more than that if you like. If you celebrate St Nicholas Day on Dec 6, then find a fabric that fits (Santa, an angel, religious symbol, etc). If you observe Solstice, perhaps choose a fabric with sunshines. If any of your family members have birthdays during December, make a birthday fabric ring. Our wedding anniversary is in December, so next year I plan to put in a fabric with hearts for that day (I didn't think of it early enough to get it done for this year). I also made a few extra links of an autumnal fabric, which I can use for the last few days of November (since we like to start our Christmas countdown right after Thanksgiving.
2--Cut out the rings. I used the extremely elaborate pattern of an envelope. A regular, legal-size (4x9") envelope. For the 25 days of December, you'll need 12 of one color and 13 of the other color (plus any November links you want, and minus any special other day links you want).
(Press over seam allowances if desired--see note at step 6)
4--Fold each piece in half lengthwise, right sides together, and sew the ends closed. Use about 1/4" seam allowance (the edge of the presser foot) and then you won't have to trim them. Be sure to backtack at the fabric edges. I recommend just sewing one right after the other, without breaking the thread between. It saves thread, but it also is a lot less work than doing each one separately.
5--Cut apart all the links. Trim the corners of the seam allowances on the folded side of the fabric (no need on the open side). If you don't do interfacing, you can get away with skipping the trimming.
6--Turn each piece right-side-out. Tuck in the raw edges, and topstitch the link closed along that side (again I recommend sewing one after the other, without breaking thread between. You may want to use different colors of thread for different colors of fabric, as this topstitching will show.
Note ~ If you're a somewhat experienced sewer, you can just freehand this. If you're nervous about keeping even-sized seams though, you can iron the seam allowances over back between steps 3 and 4.
So here are a whole bunch of links. If you're smart, you'll iron them at this point. Lazy me didn't...but I should have...
7--Attach the closures. The tutorial I linked at the beginning demonstrates velcro, I used snaps because I have a snap press (or you could handsew snaps or buttons). The type of closure can be whatever you like, the important thing is to remember to make sure that the two sides are faced opposite directions, so that they will meet up in a pretty loop.
9--You can put numbers on them if you like. This tutorial for a more traditional advent calender has directions for making nice embroidered numbers. You can also use a fabric pen, applique on the numbers, or leave the chain without numbers. I opted to make little tags, which I can hang to the inside or outside.
(and when the baby yanks on this chain, it doesn't break!)