To assign a keyword to a flow:
(1) Navigate to the "flows" tab and choose a flow.
(2) Select "Edit" from the gear icon in the top right portion of the flow editor.
(3) Add your keyword(s). Note that they aren't case sensitive - REGISTER, Register and register will be treated the same.
You can check the "Ignore triggers" box to prevent your contacts from triggering other flows while active in the flow you're editing. When this box is checked, a contact can't enter a new flow if they send a keyword assigned to a different flow.