Add a category
Create a new coin and give it a budget. There are five steps, and the last one is required.
On the Budget screen, find the dashed circle labeled Add Category at the end of your month coins, and tap it.
Name the category, pick a color, pick an emoji, then enter the MONTHLY BUDGET, tapping Next through each.
The last step is subcategories, and it is the one that catches people: Save stays inactive until you add at least one. Tap + Add subcategory, type a name, tap Add. These are the kinds of thing you will log against this coin, and Carlo needs at least one before the category can exist, which is why the button looks broken if you skip it.
Then tap Save and the new coin joins your Budget screen.
That last step also carries Mark as giving, if this category should count toward your giving percentage. You can change it later from the coin's Edit tab either way.
For a year category, scroll past your month coins to the year heading and use the Add Category coin down there instead. The flow is identical except you enter an ANNUAL BUDGET, the figure for the whole year. A category is either a month one or a year one, never both.
To add a subcategory to a coin that already exists, press and hold it, tap Edit, and use the dashed Add tile in the subcategories section.