Write down all of the contests of each box along with the box number on papers. This seems like a cumbersome step, but a master list becomes really handy when you need to find a specific item, especially if some of your boxes will be completely out of view.
If you prefer to keep things digital, there are even tools you can use to dictate text so you don’t have to type as you go. And if you want to take the organization one step further (or don’t trust your handwriting), you can print each line from your inventory to create custom labels for your boxes!