Advice on introducing new dog to our cat?

We have a 5 year old Siamese who is very loving. We adopted her when she was 3 so we don’t know if she’s ever been around other animals. She’s great with our 4 year old. For awhile we went through a period of her getting frequent UTI’s so we were constantly at the vet & switched her to a urinary cat food. The vet thought it could be possible she was smelling the other stray cat outside who sometimes comes around, so she would mark her territory inside on beds, rugs, etc. She hasn’t done that in about a year or so since her last UTI.

We are thinking of adopting a full grown German Shepherd, it’s a female who needs to be rehomed and has experience with children. However, shes never been around cats as the owner is allergic.

How can I introduce them to each other without our cat marking territory and for them to not attack each other?