If you exile a card to Werewhat's enters ability that is already a double-faced card, you may choose either face of the card to serve as the back face of Werewhat
Ive also heard of people including their custom designed cards in Cube, which sounds like one of the best places for it
You can use it for artifact abilities like Clues and Foods, or something entirely different like exhaust costs or pump abilities
There is a simple ratio, usually given in pre-release packages
1x Foil Alt-Art Chatter of the Squirrel 1x Foil Alt-Art Krosan Beast 1x Foil Alt-Art Squirrel Mob 1x Foil Alt-Art Squirrel Wrangler 1x Foil Alt-Art Swarmyard 1x Foil Squirrel Token MTG Arena Redemption: One (1) single-use code for one MTG Arena We Hope You Like Squirrels Themed Sleeve (may be available later for separate purchase in MTG Arena)
Every time a creature attacks, it gets +1/+0 for that turn, and every time you play a land, you get another attack