To start using the AUR package simply add decman to decman.aur_packages. To ensure a smooth change, add decman-git to ignored_packages during the conversion.
Decman is a declarative package & configuration manager for Arch Linux. It allows you to manage installed packages, your dotfiles, enabled systemd units, and run commands automatically. Your system is configured using python so your configuration can be very adaptive.
A complete example is available in the example-directory of this repository. It also serves as documentation so reading it is recommended.
To use decman, you need a source file that declares your system installation. I recommend you put this file in source control, for example in a git repository.
Decman can be used to install pacman packages. Decman will install all packages defined in the source and remove all explicitly installed packages not defined in the source. You don't need to list dependencies as those will be handeled by pacman. You can set packages to be ignored by decman, so that it won't install them nor remove them.
Please keep in mind that decman doesn't play well with package groups, since all packages part of that group will be installed explicitly. After the initial run decman will now try to remove those packages since it only knows that the group itself should be explicitly installed. Instead of package groups, use meta packages.