To mint a Sneaker you need:
Open the 'Inventory' tab,
Tap on a Sneaker you would like to mint,
Open the menu by clicking the '⋮' in the bottom right corner,
Choose the 'Mint' option in the opened menu,
In the following screen, press the '+' button,
Pick a second Sneaker that will be minted and press the 'Confirm' button at the bottom,
Check the mint price at the bottom of the screen and press the 'Mint' button.
Keep in mind that each sneaker can only be used a maximum of 5 times in the minting process. It takes 72 hours before you can mint again.
The sneaker receives a 15% penalty on all attributes after minting, but the price of the next mint is reduced. The penalty is stacking up to 75%.