This isn’t an exact science so you’ll have to experiment but here’s 1 way to try to substitute cottons for knits. On the back of pattern envelopes that are specifically for stretch and knit fabrics, you’ll see a measuring line where they say “Pick a Knit Rule”. It usually shows about 4 inches in white and part in another color. The “Pick a Knit Rule” works like this:   For knits, take a length of the knit fabric you’re interested in (a pieice that is as long as the white part) and stretch it against the measurement line. It should stretch at least to the end of the colored line. If it doesn’t stretch at least that far, then the knit is not stretchy enough for the pattern so you will need more fabric or pick something else that that meets the pattern’s needs. Different patterns will have different stretchiness requirements so make sure you use the “Pick a Knit Rule” on the pattern you’re buying fabric for. For cottons and fabrics with little or no stretch, do the same measuring noting the difference you need for the stretch.  You also need to look at your pattern to see where the fit is going to be really important (fitted waist) vs drape of the skirt.  For the fitted parts, you will need a proportionate amount of extra fabric fabric based on your measurements.  For the drape, you’ll need decide how much drape you want and add that much more.  It could be as little as a quarter more to 1/2 depending on the “stretch” the pattern is designed for (lycra vs knit). It’s not impossible but I’d suggest getting some super cheap cotton fabric or muslin to make sure you get the proportions that you want before making your finished garment.  In either case, you’ll get an extra garment you may wind up loving too.  (Dye the mulin and/or embellish it with fabric paint, trim, etc to make it more wearable). You might also consider expanding your world.  Choose a cheap knit fabric using the “Pick a Knit Rule” and go for it.