A lot of referees feel a little lost when running their first sandbox campaign. Feeling overwhelmed by the need to generate dozens of locales and having no idea what the PCs ultimate objective will be.
A good way to over come this is to make a sandbox setting but have a campaign where the PC are doing missions at the start. They are all members of the City Guard, one of the Guilds, or the King's Court. The mission helps focuses your preparations. Then let the players have free reign to decide how to accomplish the mission.
Then follow up on the consequences of their decision both bad and good. The follow up is important because this establish the sandbox nature of the campaign. Several sessions in, the players find themselves driving the campaign as they follow up on the consequences of the consequences of the consequences of ... well hopefully you get the picture.
By this point you will have learned what tricks and techniques work best for you. How to best use your (limited) prep time.
D&D 5e Monster Manual: Black Dragon Awesomeness
35 minutes ago