try and imagine you didnt have a tickle fetish,
it wouldnt seem weird to tickle someone then....
its a playful thing that everyone does, 9 out of 10 people will tickle a bare foot if its within reach and theyre feeling playful, its the norm, its one of the most commonly and widely known things to do with a foot anyway,
no trickory is needed, just patience,
as mentioned above somewhere, if shes not bare foot much, then perhaps the foot rub idea will help you, just wait for her to complain about her feet hurting, (which ALL women do at some point) and offer a "helping hand" it will most liekly be seen as a kind gesture.
if she doesnt say anythign about it, then perhaps just start randomely talkign about massages, and it may pop up that way,
its a game of manipulation my friend,
get out there and "cause" 🙂