Perform the following steps:

$ git clone https://github.com/mmgen/mmgen-wallet.git
$ cd mmgen-wallet
$ mkdir -p ~/.mmgen
$ cp nix/user-packages.nix ~/.mmgen

For altcoin support, edit ~/.mmgen/user-packages.nix to taste, uncommenting the relevant lines for whatever support you require. For a BTC-only setup, you can leave the file untouched.

Build required derivations and enter the custom Nix shell environment:

$ nix-shell nix # for full isolation, add the ‘--pure’ option

Within nix-shell you may now test and use MMGen Wallet as with any normal installation.

Refer to nix/README in the MMGen Wallet repo for more information, including a speedup tip and instructions for installation on NixOS.