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You can set up your VAT now by turning it on within the Taxes tab - here you can enter your VAT registration number, choose the start of your VAT quarter, and set your filing frequency and vat accounting scheme.
Once you have turned tax on you will be able to view the custom VAT codes by going to Edit VAT > Edit Rates. Then, when you are ready to start using VAT, simply assign the VAT code when entering the transactions.
You will see the new field for VAT on all transactions as soon as you have turned VAT on. To remove the column so that you don't enter VAT until you need to, go to the Gear icon > accounts and settings > advanced > change the default tax rate selection to 'No VAT', then change this back when you begin using VAT.
You can see more on setting up VAT in QuickBooks on our community guide here.