Choose key combinations that don't already have assigned actions.
A word of warning: Do not start assigning commands willy-nilly without checking first what keyboard shortcuts already exist; you could destroy some existing keyboard shortcuts. There is already a vast selection of keyboard shortcuts built into Word, so find a key sequence that hasn't already been assigned. When you enter a keyboard shortcut you are told if that key sequence has already been assigned to an action or you can find a complete list of keyboard shortcuts in the Help topics.
Note Although one action can have many different keyboard shortcuts, each key combination can be assigned to only one action.
Think carefully about what key combination you want to assign to a particular action. Use a sequence that you will remember and associate with the action. Do not use shortcuts that are all similar and not related to the action, unless you are certain that you will remember them.
Also, beware of saving different actions using the same key combinations in different documents and templates. The temptation is to reuse the same shortcut combination with different actions attached. But it could be very difficult to remember exactly which actions you have saved in each separate document.