When you greet them at the door just motion or point to a spot on the floor and politely say "you may leave your shoes there". Make sure there are other shoe there so that your guests get the idea. Do not request them to remove them outside and leave them outside. That would be rude.
You can simply ask your guest to do so while you are at the door inviting them in. You can also put a sign at your door noting the same; however, ensure that the note is written in a comical manner in order to avoid hurting people's feelings.
All of the above suggestions will probably work, but just keep in mind, that if the police come to your home responding to a domestic disturbance call, they are probably NOT going to comply with your request. (Just so you know.)
I am not saying that this rule is bad. But asking someone to remove shoes especially a guest is rude and very inappropriate , no matter how politely you ask.No one wants to follow your rules unless they have too.
I have a friend too, who have this rule.I stand outside his house never go in. I am sorry if I am rude, but if other person, do not have the same rule, he will neither respect it nor like it.
We just put a sign on door. Thanks for removing your shoes. You can keep a few pair of non slip slipper socks and a shoe caddy out side the door. The only people allowed shoes is my mother. Even contractors take them off or I puts few towels for them on floor if they go through living area to basement
Sometimes taking shoes off in the house is a cultural thing. Usually it is because someone comes from a country with bad sanitation. If their home country has open sewers or dumps their waste outside of the house, they probably prefer that you take off the shoes. Nobody wants filth tracked into the house.
Just take off your shoes and be polite. This can backfire if you have smelly feet though. Laugh and ask if they want to smell your feet.