Short answer: you don't. At best, you can use the OTP to secure access to a key stored somewhere.
OTPs are for authentication, not encryption. You can write software that checks a user's OTP before granting access to data. This is just standard authentication, such as the way that Github (for example) checks your OTP at login before granting access to your account; it won't encrypt the data. If you want the data to be encrypted, you need somewhere to store the key.
OTPs are far too short to use for key derivation and only briefly valid anyhow (and intended only for one-time use - it's right in the name - so not suitable for something where you'd need the password again like re-deriving a decryption key). You either need to store the key somewhere trusted (and then restrict access to it, possibly using the OTP as an authentication mechanism), or you need to use a longer, multi-use password or similar user credential that can be run through a key-derivation function to produce the encryption/decryption key.