This is a bit tricky to figure out as some shortcuts are already taken by Office. Press and hold "ALT" to view current shortcuts, try pressing a letter next, if nothing happens that key is available.
Shortcuts in the OfficeUI file must take the form of "ALT+foo+bar".
This sets the "TAB" shortcut to "ALT+L"
This sets the "Button" under the above "TAB" to "1" creating the final Keyboard Shortcut of "ALT+L+1"
Note: The way I generally do this is use the 'Customize Ribbon' wizard and then edit the "Excel.OfficeUI" file directly.