1. Tap the Transactions 
button to reveal your transactions.
2. Tap the transaction you want to split.
3. Tap Add a split.
4. Tap on the Budget Items for the split. Then tap Done in the top right.
5. Enter the split amounts for all the budget items you selected. Make sure they add up to the full transaction amount. You can see the amount remaining on the bottom right.
6. Tap Save.