Doing a petticoat is pretty straightforward. I'v done my first one with a tutorial I found online years ago and have since lost the link to.
I'm talking here about a tulle petticoat meant to be worn under a circle skirt type of dress to make it look fuller. It is not the same as a tutu/ballet skirt which is worn as is and goes straight out from the hips.
So, how do you make one, when you've lost the tutorial? Here's how. Measure the widest part of your body (your ass).
Measure how long your want the petticoat to be. Cut that measurement in 3 to 4 parts depending on how long your petticoat is meant to be (longer = more sections). A knee length one can be done with just 2 strips, but I do prefer mine to be a 3 sections one as It gives it more fluffiness.
Back to the first measurement. you're going to want the first section, the one closest to your hips, to be twice the size of your initial measurement. The second layer will be twice that, the third one twice that of the second, etc.
ex: 1st layer : 7'' x 94''
2nd layer : 7'' x 188''
3rd layer : 7'' x 376''
So you cut out a bunch of long ribbons of tulle with the height you've got (for me, the 7''). Then you grab the many pieces and you start assembling them together to make the sections.
So here's tip #1: To make your life easier do not try to calculate "376 inch ". Instead roughly estimate how long one strip of fabric is and calculate in number of strips.
ex: 1st layer : 1 1/2 strips
2nd layer : 3 strips
3rd layer : 6 strips
Tip #2 : Directly in relation to tip #1, do not make one long strip and then cut it down. Having it that long means more chance of stepping on it and ripping it. Instead make each strip for each section you need as you go.
Tip #3 : Tulle is shifty and sticking a needle in won't help in any way except you getting it stuck in your feet. Instead get some quilter clips. It will save you the headache.
Tip #4 : When assembling your pieces, a straight stitch will do you no good. The best is to use a Serger but I understand you likely don't have one. Instead, for a quick Serger replacement, use a double needle*. It will make both a zigzag and a straight stitch and keep everything put. If you forgot to grab one or are unable to get one in time, do use a straight stitch but be warned you will need to use a french seam technique*.
Tip #5 : You don't want to have to remove stitches from tulle. It only makes a big hole of a mess. in case of accidents just use your scissors and redo that one seam. It will be easier to keep your head cool. But most of all do your best to not have to remove any stitches from your tulle. That means stay calm, take your time, do the things right first. If you have to run your fingers twice around to make sure your section isn't stitch back to front creating a weird loop, do it.
You have sewn all of your strips to the right length into circles. You now have to stitch those together. So set your machine at the longest straight stitch you can do and run it on the whole length of the section without stopping the ends. Leave enough to grab of the thread on both side. Pull and gather it until it is the size of the section above then smooth out the gathers so it's even everywhere. Run it in the sewing machine again with a much smaller straight stitch to keep the pleats where you want them.
Tip #6: At this point always start from your largest section until you reach the smallest section. Otherwise you'll be trying to attach a gathered piece to another already gathered piece. While instead you can gather the bottom, attach it, then gather the second piece already attached, etc. of course you need to run your gathering stitch first to be able to do that easily. But it's more than doable. it just takes patience.
When attaching the sections together, again, use either the french seams or the double needle. Now, all that's left is adding another layer(or two, or three) by doing the same thing all over again. This mostly depends on how fluffy you need your dress to be and how heavy the fabric is.
After all that work, add a waistband or an elastic. and you're ready for your lolita meetup.
Just throwing in two extra tips.
Tip #7 : If you plan on hemming the largest section at the bottom with a trim or bias tape, do it before you start gathering and assembling the sections together.
Tip #8 : Use the thickest, most rigid, tulle you can find if you don't want to have to make 4-5 layers of petticoat. The thinner one looks better but it doesn't hold up as well. If you MUST use it, make one layer of thin tulle and 1-2 of thick one. Sometimes cosplay asks for that level of detail so go for it.
Hope this helps !