Install MathWallet https://mathwallet.org
Switch Network to 'Moonriver', create or import a Moonriver wallet
And you can start using Moonriver now!
If you need to claim your MOVR token, using your App scan the qrcode of 'Moonriver Apps' on https://mathdapp.store/?blockchain=moonriver
And you can claim your crowdloan MOVR token, same as the extension process
https://blog.mathwallet.org/?p=3341