You can use the app itself for this.
Just create your hunt in skeletal form, filling things in as it gets clearer to you. You can add and remove clues, rearrange them by dragging them up and down, and change each one’s image style and other attributes as you go, so you don’t need to get the whole hunt picture perfect the first time. For example, you could create a bunch of “Location or Question Only” clues as placeholders, and then as you fill them out you can change them to “Match Image”, “Describe a Task”, or “QR Code” by tapping Change Type when editing a particular clue.
If you’re using GPS coordinates, you can also update locations for each clue, so you don’t have to get that right at first either. That’s under More Clue Details->Adjust on Map. To put a pin where you currently are, tap the location arrow in the lower left of the map to turn on location, then tap the “move pin to current location” icon at the lower middle of the map:
Also, you can use the Notes field for each clue to help you out (under More Clue Details for each clue). This isn’t shown to players, it’s just for the creator of the hunt.