The best way to go about this in my opinion is to look at what you're currently developing and see what assets you need for it, then hire people to work on those specific assets.
Let's say you're starting a 2D game and need a tilemap for a cave. A main character that can just move for now, your first monster with movement (since we're not working on a combat system yet) and maybe a main music loop. Create a spreadsheet of the people you need and their price, for example:
| Asset | Link to Fiverr | Cost |
| Cave Tilemap | John Doe | $30 |
| Main Character Sprite Sheet (Movement Only) | Jane Doe | $50 |
| Slime Sprite Sheet (Movement Only) | Jeff Doe | $30 |
| Music (8-bit track - 2 minute loop) | Josh Doe | $25 |
| TOTAL | $135 |
Now you have your base game assets which you're going to need to build out the layout for, create the cave scene, set up the sprites and creature AI for etc.. All that is going to take you some time. During that time plan out your next system, maybe it's the combat system, list out what you need for assets and save up for the cost.
Keep in mind each person will charge differently based on what you need and the complexity of it and each person will take different amounts of time too. You might get your music track in a week, but the tilemap in 2 weeks. - At least you're paying for a final product and not hourly for an undetermined amount of time.