I've never cleaned this, no need, IMO.Periodically I throw in a splash of plain bleach and some water. Give it a good shake and rinse. I'd hate to say how long I've used some of mine but always kept sanitary! It's getting harder to get the ones with the screw on caps instead of the pop on. Times they are a changing.
I actually hacked the neck so a screw on cap would work,
at least well enough that if it tipped over it won't spill much.