Goals for this dress: copy a beloved favorite top that I often wish was dress length. Learn how to make pockets.
My best advice for anyone looking to start making their own clothing is to go to the thrift store* and buy a sheet. Or better yet- a duvet cover. Muslin is wonderful, but expensive- with thrifted yardage you'll have a wealth of not so precious fabric to experiment with. Even if you wind up hacking the whole thing to pieces without producing anything wearable, you'll have scraps for a quilt or a massive stash of rags for wiping the bacon grease out of your frying pan.
Pick a sheet that feels nice- natural fibers are always a bit more of a joy to work with- look for a high cotton content, or even linen if someone was feeling generous in their donating. You can cut around stains (or incorporate them? Ew?). Stripes and plaids can work as a sort of grid to help you cut straight and to measure. Don't worry about colors much- if you wind up with a wearable garment you can always dye it. (Note: I dyed this dress in my indigo vat, but RIT dye is readily available most places, and I'm sure black tea is well within your reach).
I draft my patterns with a mix of sort-of-laying-things-out-and-tracing-plus-eyeballing. Think about how you like to sew- do you follow patterns to a T? Cut up an old garment you love and make yourself a pattern from it. Are you an eyeballer and an adjust-later-er? Do that. Look at garments you love and write down the steps you think it took to make them. Skip the fussy steps if fit is what you're trying to perfect. Practice French seams and miniscule pleating on a box top, if finesse is your goal. There are no rules, and you can only learn from your mistakes, and you have that whole damn sheet to learn with, so just go for it!
* Does the thought of buying used bedding at the thrift store sketch you out? Don't let it! In my experience, good thrift spots are usually close to decent laundromats, so for a couple dollars more you can launder your new yardage outside of your home and bring it in clean and ready to go!