First, would you excuse me if I make some gramatical mistakes, I'm not a native english speaker. I've done my best.

You will need :

20 inches BMX metal rim in good condition
2 gear motors
2 M6x15mm hexagonal female thread spacer + M6x15mm screws
6 3D printed pieces "rim.stl"
2 3D printed pieces "motor.stl"
2 GT2 20 teeth 5mm pulley
125cm GT2 belt
6 M3x6mm screws
28 M3x20mm flat head screws + nuts
12 M3 washers
6 M3x15mm (classic screws)
Electric wire
2 battery connectors ( here a JST and a XT6 were used)
12V battery (here a 3S 2200mAh LiPo were used)
A ON/OFF switch
1m boning
1m bias
4 metal press studs
1x0,3m solid fabric or fabric+interfacing (waistband)
Zip or hooks
1m fabric (waistband cover)
2,5m piping (optional)
1,3m solid ribbon
1,5m lightweight fabric (skirt)
1m lightweight cotton fabric (underskirt upper part)
2x1m tulle (underskirt lower part)
Approx. 2x18x0,15m tulle (petticoat, lower part)
Approx. 18x0,2m tulle or satin (petticoat upper part)

And some tools such as :

Angle grinder
Drill
File
Screwdriver
Neoprene glue
Silicone spray
Sandpaper
A sewing machine

Step 1: Prepare the Rim

RECAP, you will need :

20 inches BMX metal rim in good condition
125cm GT2 belt
28 M3x20mm flat head screws + nuts

For this step you have to cut in half the rim just under the angle (second drawing). Be carefull to cut very straight. Then clean the cut and the outer plane surface of each new part with sandpaper. These plane parts will have to slide one on the other (See the third drawing).

Once you've done with the first tricky step (only two or three, don't worry) you must mill the holes of the spokes on the inside (fourth drawing).

Place the M3x20mm flat head screws + nuts. Screws must not dislodge since then. Clean a little bit with sandpaper.

Then glue the belt nearest from the rounded edge on the same side of the milled holes (fifth drawing). Be carefull to glue it on the plane part and to be VERY VERY REGULAR!

Step 2: The Guide

RECAP, you will need :

6 3D printed pieces "rim.stl"
12 M3 washers
6 M3x15mm (classic screws)

You need the 6 3D printed guides of the "rim.stl" files.

Guides have to be equally placed around your rim.

3 of them have to be drilled again at 3mm to help the screws go through without forcing.

Then glue a redrilled one with an original one on the horizontal surface of your rim (see first drawing).

Spray some silicone to help the rotation later and screw your guides with M3x15mm screws, a washer outside and 1 or 2 washers between the two part.See the third drawing to help! The washers between are there to limit the tightening excess.

Once you've done, your rim must look like the fourth drawing.

Test the rotation of your rim. it would be easy, without forcing too much and without too much liberty.

Step 3: The Motor Part

RECAP, for this step you will need :

2 M6x15mm hexagonal female thread spacer + M6x15mm screws
2 3D printed pieces "motor.stl"
2 GT2 20 teeth 5mm pulley
6 M3x6mm screws
Electric wire
2 battery connectors ( here a JST and a XT6 were used)
2V battery (here a 3S 2200mAh LiPo were used)
A ON/OFF switch
2 gear motors

Each motor must be placed on the small half of the rim and in front of the other to enhance the rotation (see the first drawing).

Place your plate on each motor following the first drawing but not the pulley! Place a M6x15mm hexagonal female thread spacer + M6x15mm screws on each side of the plate (see second drawing). This piece will help to secure pulley close to the rim and the belt by adjusting it.

And here comes the second tricky step : the pulley (see the fourth drawing)

Since the pulley can't be placed on the motor due to a thiner hole, you have to enlarge it to 5,5 mm with a drill or every usefull tool.

Be carefull and patient and firmly hold the pulley.

And since the external parts of the pulley are too large, it can't touch the belt, so you have to reduce the upper and lower parts to 13,5mm with a file.

And once again be carefull and patient and firmly hold the pulley.

And it's done! You can place your pulley and be proud of your meticulous work!

Next and last tricky step : a hole in the rim

To place the fourth M3x6mm screw of the motor on the rim you will need to drill a hole with two different size on the upper (3mm) and the lower (6mm) parts of the rim. See the tenth to thirteenth drawings (I'm sorry I've made the tenth drawing upside down).

Now you can place your motors on the rim!

Now motors need to be wired in parallel. Be carefull to make the motors turn on the same direction. As seen on the photo number 16, a connector is directly weld to one motor. Then you could prepare the switch and the battery connector (see photo 17).

And right now you could make your future skirt turn for the first time ( and testing your system in the same time).

Don't hesitate to make it turn for a moment to check the slide, add silicon, tighten screws or adjust some little things.

Step 4: The Skirt

RECAP, you will need :

1m boning
1m bias
4 metal press studs
1x0,3m solid fabric or fabric+interfacing (waistband)
Zip or hooks
1m fabric (waistband cover)
2,5m piping (optional)
Eyelets or Zip
1,3m solid 3cm-wide ribbon
1,5m lightweight fabric (skirt)
1,5m lightweight cotton fabric (underskirt)
2x 2x0,5m tulle (underskirt)
Approx. 2x18x0,2m tulle (petticoat, lower part)
Approx. 18x0,2m tulle or satin (petticoat upper part)

The waisband :

Follow the cutting directions of the scheme joined "Pattern.pdf".

For this project I use piping to add details. To do the same cut your waistband cover in four and sew it with the piping on.

For the waistband, you must use a solid fabric or add interfacing as it has to hold the rim. Fold your band in half (right side face to face), sew the long length and flip it to the right side. Then sew the waistband cover. Use hooks or eyelets or a zip to the closure (see the third photo).

Now, this is how to tie the rim to the waistband! You could use 4 bonings (for corsetry) covered with bias fold in half and sewn. At one edge place a metal press studs (those to crimp across the fabric) (Photos 4 to 6) and just present/place the other part on the rim (equally placed aroud the rim). Then, equally sew these little band on the waistband and strongly glue the other part of metal press studs (photos 7 and 8).

The skirt and underskirt :

Once again, follow the cutting directions of "Pattern.pdf". Sew edges of each piece and sew together the waist length of the skirt and underskirt. Sew the 2x0,5m tulle strips to a 4x0,5m one, gather and sew it at the middle of the underskirt length. On the right side (to be between the skirt and the underskirt).

For ruffles, no miracle method but use very long strips of fabric/tulle/satin (at least 2,5 times your finished length) and make the ruffles until the desired length. Sew one strip on the underskirt and the other on the tulle. Then sew the larger one (here the red satin) above the previous strip on the tulle underskirt.

You can also make some wave like the nineth photo to make the ruffles appear.

Now, it's time to attach the skirt to the rim!

For this, you will need the 1,3m 3cm-wide ribbon. Place it on the outside of the rim, just above the end, and make holes at each screw. See the tenth photo. Sew the edge together and then sew this ribbon to the edge of your skirt. Place your ribbon before the nuts and...

YOU ARE DONE!!!!!!

Wow, this tutorial is finished. I hope you understand my words and explanations. Don't hesitate to ask me everything you've not understand.

And please, let me see your version of this turning skirt!