First stack both your washers one on top of the other and measure round them - this measurement doesn't need to be really tight but the thickness of the tape measure and the fact that it bends at the edges, seems to give enough extra room for the 'pockets'.
Make a note of the m/m (or, if you have a better memory than me, just remember it!). Mine measured 4.5", although in the photo the tape had slipped a bit so it looks like more.
Next make sure you have a rectangle of fabric that is large enough to wrap round the weights with a 'bit' to spare. I was using a scrap of leftover quilting cotton and it just happened to be the right size - incidentally, I could have made covers for 8 weights out of one width of fabric (112cm/44"). If you want to be exact you can double your previous m/m and then add a bit for seam allowances but it is easy to eyeball this. You will need extra fabric on the three raw edge sides of the weight pocket, the fourth edge will be the fold of fabric.
Take the m/m from the first step (or, if you have forgotten it, measure again Lol!) and halve it. Fold your rectangle of fabric right sides together and mark that measurement on the fabric, centre it so you have a seam allowance on each side but it doesn't matter if they are exactly the same.
Stitch the seams down both sides on the marked lines. Reverse at the folded edge for strength. Trim the corners at an angle and trim the seam allowances to approx 6mm/1/4".
Turn your 'pocket' through to the right side. Slide your two washers in, they should be a snug fit but not so tight that you are forcing the stitches apart at the seams.
Tuck the leftover fabric in at the top. This takes a bit of fiddling with, I used the blade of my scissors to push the side seams down as that is where the bulk is. It should look like the picture below.
Stitch the opening closed with a hand stitch and matching thread, only takes a minute!
Finally, as an optional extra, stitch a button on through the hole in the middle of the washers, this also tightens the 'pocket' round the washers. I just used a stranded thread, Madeira Decora, and left the ends to tie.
This is the thread I used and the huge needle!
Finally, after about half an hour....................Ta Daa! Six new fabric weights! I love these as they are nice and flat and don't get in my way when I am cutting out.