Or under the Trade tab, click Currency Exchange ^^ You can buy diamonds with gold from there

do I write an outfit name, click "Create Outfit", then put the outfit together and click "Save Loaded Outfit" ?

Yep, you can do that~ ;3

And if you don't like the name or just want to change the Outfit you saved for something else, you can always re-name it~ :D

oh crudsicles, I think I accidentally used up my second slot on the same outfit as my first T_T

It costs nothing to save a new outfit in that slot. c:
So easy to fix mistake~

I'm trying but it's not working, I don't understand, what am I doing wrong ?

Hmm...Ok, so first you'll want to be in the slot of which outfit you want to change. Easy enough to do that, as you just select it from the little drop-box and then hit "Load outfit."
Then just mess around in your wardrobe and create your new outfit, and then just click the "Save Outfit" button!

I'm so confused, it won't let me change my outfit

You have to own some of the outfits, the ones you don't own will be highlighted blue and the ones you do own will have white BGs. you can preview outfits you don't own, by pressing the magnifying glass.

how do I own what I want to wear ? like how do I get it ?