Running ReaScripts
Save scripts to the REAPER/Scripts resource directory and load them via the Actions window.
Kind: how-to
You will need to either write a new script, or load a script from another user. Scripts can be placed anywhere on disk, but it is convenient to keep them all in the REAPER/Scripts resource directory. To find the resource directory: Options > Show resource path in explorer/finder.
To create a new script, show the Actions window (bound to the ? key by default), and click ReaScript: "New...". You will be prompted for the location to save the script (the default location is the recommended REAPER/Scripts resource directory). The type of script will be deduced from the file extension you enter (.eel, .lua, or .py).
To load a script, click ReaScript: "Load..." in the Actions window, and select the script file you want to add to the action list (multiple selection is allowed).
Once you have written or loaded a script, it will be added to the action list, and you can treat the script just like any custom action: run it from the Actions window ("Run" button, or double-click), bind it to a key shortcut, mouse wheel, MIDI/OSC controller, or toolbar button. You can get binding information at runtime too, details here.
ReaScripts can also be run à la carte, via the actions "ReaScript: run..." and "ReaScript: run last script".