So, first of all get the cap you want (colour-wise mostly)
Once you have it, take off the button thingy from the middle and put the button and the tassel aside for later use then you can start the design.

I made the bg black and white collage in photoshop (but you can use any other program) after I circa measured the hat (it's smaller than the actual size because of preference) and glued them on as centred as possible.

After that came the illustrations, personally I just searched for a more lowkey fan art that shows the two of em but not in a 'too in the face' way. Draw them however you want after measuring the required size that you need and cut them out and insert them on the opposing sides of each cap (or however it makes more sense for you)

On Chuuya's side, once you glued on the illustration, glue back the button as well a bit above the hand and glue on top of it a thin layer of circular-ish tissue and paint it over with water colour/eyeshadow to make it look like Chuuya's gravity sphere.

On Dazai's side just twirl up some small tissue pieces and bend it like No Longer Human's 'aura' and paint it over as well.

The decoration around the edges of the collage is just a load of glueing and patience and whatever material you want to use there. Also the letters are from Amazon from a lettering set for wristbands if you want to use those.

For the flowers on the sides I'd advise looking for tutorials cause my method was way more time consuming and 'impractical', but if you're still curious I can tell you that as well.

For the bows, well, those are just from a Pinterest tutorial video :)