There are a couple of ways you can create a Multi-Account Budget:
Setting up a Multi-Account Budget for a newly imported Ad Account
Setting up a Multi-Account Budget from an existing Ad Account
It is not possible to combine two Multi-Account Budgets. Ie. Budget 1 includes Google and Facebook, and Budget 2 includes Microsoft and LinkedIn - these cannot be merged into one budget.
1. Setting up Multi-Account Budgets for a newly imported Ad Account
When you are setting up budgets for a newly imported Ad Account, you can create a Multi-Account Budget as the first budget you create.
At the top of the budgets page, there will be a notification advising you that you have X ad accounts missing budgets - click this to open a modal.
From the modal, click 'Create a Budget' next to the Ad Account. This will open the 'create a budget' page.
Once you have added the Ad Accounts, you can follow the standard settings when creating a budget.
2. Setting up Multi-Account Budgets for an existing Ad Account
For each Single Ad Account Budget, there is a triple-dot option to 'Create a new Multi Account Budget'.
This will create a brand new Budget where you can set up a Multi-Account Budget from scratch. The existing Budget will be removed by making a Multi-Account Budget from it.
Once the blank Budget page opens, you can add Ad Accounts to this new Budget.
Note: You can only combine Ad Accounts that are not already in a Multi-Account Budget
Once you have added the Ad Accounts, you can follow the standard settings when creating a budget.