I tried another similar Step stool and found it would not stay together. This stay together.

I had not tested the capacity totally. It will depend on your print setting and material. I made mine with 4 outer layers 15% infill, using PLA. I weigh 240lb (109kg) an was able to jump a few inches.

I printed my logo in the side. This is not on the files uploaded.

After the printing,

1. Move the joints to make sure they are flexible, after print and before assy.
2. insert the corners in the top. as shown in the attached photo. The first side will go in easy. The second side will need more force to get together. This helps the parts stay together. These pins have a flat side and will go together in a flat position (the top is flat with the side). When the sides are locked together, it will not come apart.
3. Snap the end “hinge” parts in place. the knuckle of the hinge goes outward.
4. use the small rectangular pieces to lock everything together.
5. You need to print two copies of some parts. The file name ends in x2.

There are 2 sizes of stools, 1 that will print on a printer with a 200mm (7.9in) max print size and a second with a 256mm (10.0 in) max print size.

Depending on your printer setting, you might not need support. Printed fine with it on my X1C printer without support.

I added a file with larger gap (50% bigger gap). to make it more friendly to uncalibrated printers.  

you still need to clean out the lock holes. (the holes for the flat parts to slide into that lock it together.

I also added some non printed text to the plates that reminds multiple prints.

The looser file is:
StepStool3 full set loose.3mf

the default is the tighter version unchanged:

StepStool2 full set.3mf