Maybe don't start with building the entrance? Leave it very simple until you know what the rest of the zoo will look like?
I play franchise so I usually build some habitats first and decorate the entrance later because of limited funds at the start. Also it seems easier because the zoo is already running and I can get busy adjusting things and getting ideas about how to integrate it with the rest.
But sometimes I just draw quite detailed plan on paper, get about 50% of it done in the game and then I have to improvise because it looks nothing like my initial plan