a ruffler foot or a gathering foot can be helpful (if you can find a good one for your machine) but I have done several by hand as well. you need to decide how many tiers you are willing to do and what your hem measurement will be. honestyl, 25 yards is a lot of gathering for a first attempt, and a 10 yard skirt is still very full.
so- measure where you want the waistband to fit and add 5 inches (A). you will eventually cut one piece of fabric from your waist band fabric that is 5 inches by (measurement A)
measure from where the waist band will sit to where you want the hem to fall. the waist band will take 2 inches of that, but we will also allow 2 inches for the hem and mistakes, so we break even.
round your measurement A to the nearest half yard and add a yard (B) this is your first tier. now you know the measurement of the first tier, and the measurement of the last tier- for the sake of explanation, let's say you have a 36 inch waist and we're doing a 10 yard skirt- my waist band will be 41 inches, the first tier 2 yards, the last tier 10 yards. how many tiers you want will determine the increments of increase- more tiers gives a smoother look, but the ruffling may drive you crazy. let's say you want 4 tiers for now.
first layer is 2 yards. second layer x, third layer 10. can go up to double yardage without it looking too ruffly, but go less than that if you can
options- 2, 4, 8, 10, or 2,2.5, 5,10 working from the other end
let's split the difference and go 2, 3, 6, 10- so you will need a total of 21 yards of strips- if you care about exact, add 3/4" per seam (this will vary depending on the width of the fabric you are using)
to determine the width of the strips, take the length of skirt measurement we did in step 2 and divide by 4. now add 3/4" seam allowance.
I'm short, so we'll say 32/4+3/4=8 3/4"
sew each tier into a circle (be careful, you don't want a mobius strip)
go ahead and hem the bottom tier now- I usually use the rolled hem foot on my machine for a nice tight 1/8" but preferences vary. do it your way ;-)
start with the top tier- you can use a ruffler or just do a basting stitch and gather by hand- gather so that the tier matches the length of the waistband (less 1 inch seam allowance for the waist band)
fold the waist band in half (hot dog style, right side out) & press. open, fold one edge in 1/4 and press. fold the ends of the waistband over 1/2" on each end, butt together and baste.
pin waistband and first tier (gathered edge) right sides facing and sew with 3/8" seam allowance. I like to use zigzag from here on out for greater strength, but you can do a straight stitch if you are having a hard time grappling with the gathers. fold the waist band over and topstitch folded edge to the back of the first tier.
now gather the second tier in the same manner and sew to the first tier. repeat with the third and fourth tier.
Now, a little extra work ensures a much longer life- press seams up (as in the seam will lye on the upper tier) and top stitch in place.
feed a nice, wide elastic through the waist band (for fit, wrap the elastic around where you want the skirt to sit, cinch it in so it is snug but not uncomfortable and cut 1/2" out from where the ends meet) sew the elastic together add a drawstring if you want and sew opening closed.
every time I make these things, I swear I will never do it again, even with a gathering foot, yet even now there sits to my left 48 yards of strips, half gathered :-/
good luck.

Thank you!