I've never made one, but I think you could sew a tube that is the length you want the top to be plus the additional length of the ruffle. Make sure it is a comfortable fit around your bust/hips. If the terry is the same on both sides, just fold it over to get your ruffle length, and sew the folded edge to make a casing for the elastic, leaving a small section open for inserting the elastic. Try it on, adjust the elastic to your liking, then mark or pin the elastic. Take it off, sew the elastic ends securely, stitch your casing the rest of the way, and that should be it.
If your terry is smooth on one side, you will need one or two more inches of length, and after folding over the ruffle, you would cut the ruffle length off, place the ruffle on the inside of the tube with the nap facing the inside of the tube, sew the two pieces together at the top, and flip the ruffle to the outside. The ruffle and the tube should now look the same. Then proceed as before, fitting and sewing the casing for the elastic.
Gosh, I hope this makes sense! I can see it all in my head...I just hope I explained it alright. I hope others will chime in and help make it a bit more clear.
If you look for vintage patterns online I'm sure you could find a pattern for this.

hi noinwi...thank you for taking the time to answer this
now to show you how much i dont know i measured around my chest ...above my breast ...and it measured 44 inches my question to you is how much additional fabric do i add to the 44 inches...is there a rule of thumb for stretch fabric
and dose the streatch go around or up and down
again big thanx

I haven't sewn stretch fabric in years, but I'll try to give you additional info. You want to measure your bust and your hips. Depending on your figure, you want the top to be a little larger than your largest measurement(unless you like tight fitting garments). For example, I'm pear-shaped...my hips are quite a bit larger than my bust, and I prefer my tops to be long enough to be even with my crotch(yes, I know I'm really not hiding anything LOL). I would use my hip measurement plus a few more inches, as my fabric width. The gather from the elastic on top will take up the extra fabric and the ruffle will be fuller. For the length of the ruffle, make it long enough to reach below your bust(we're going for modesty, right?)...measure without a bra on if that is how you will be wearing the top. Does this make sense so far? Do you have a t-shirt or other pull-over that fits the way you like? Try it on and note how tight or loose it is, then use that measurement for your width.
Do you already have fabric? The stretch of your fabric will be crosswise, or side to side, not up and down. Some terry cloth will have a directional nap, where the loops slant or lay in one direction, and you'll have to keep this in mind if you have to cut the ruffle separately, as you'll have to make sure it lays the same way when it's sewn on.
If you haven't purchased your fabric yet, the salesperson in the fabric store should be able to help you choose the right type. Also, if you haven't sewn stretch fabrics before, you should look online for 'how to sew stretch fabrics'. It will be invaluable.
Does your machine have a stretch stitch or a zig-zag stitch?
And you need a ballpoint needle.
And lastly, if you know someone that sews, enlist their help.
I hope I covered most of it...and I hope I didn't confuse you to the point of giving up. Let's hope someone else will chime in and correct any misinformation I may have given.

BIG THANX.....BIG HELP ....WHICH EVER IS THE LARGEST OF THE TWO
HIPS OR BREAST IS WERE YOU START... WITH ME THERE ARE ALMOST THE SAME SO I WILL START WITH THE LARGEST # AND ADD A LITTLE MORE SINCERELY FRANCES