The simple answer is – you can’t. Unless you purchase a wallet that is specially designed to carry an AirTag (like the ones listed in this article) you’re going to have a hard time safely securing the AirTag in place and will most likely cause stress, stain, and a large bulge in your wallet.
There are a few exceptions though. The first, is zipper-style wallets, that host a large internal slot. These slots have more than enough room for storing an AirTag, you just have to be aware the AirTag might be loose inside the wallet.
Another option is buying something similar to the already recommended Nomad Card, which secures an AirTag inside a plastic case making it resemble a credit/debit card. This would then allow you to store the AirTag inside a typical wallet’s card slot. It’s still not ideal, but it’s better than nothing and a much cheaper option for storing an AirTag.