When you file as married filing separately you have to file two separate tax returns, one for you and one for your spouse. There's no practical way to split a joint tax return into two separate married-filing-separately returns. Just start two new tax returns from the beginning.
Theoretically you could remove all the spouse's information from the joint return and change the filing status to married filing separately for the person who is listed first on the return. But doing that is difficult and error-prone, and could produce errors in the return that you would not be able to fix. You're not likely to be able to do it successfully. Creating a married-filing-separately return for the person who is listed second on the joint return is simply impossible.
Why are you filing separately? That almost always results in paying more tax than if you filed jointly. It makes you ineligible for a number of deductions and credits, and the tax rates are higher. Also, when you file as married filing separately you have to either both itemize deductions or both take the standard deduction. You're not allowed to take the standard deduction for one of you and itemized deductions for the other. See the FAQ at the following link for more information about filing separately versus filing jointly.
Is it better for a married couple to file jointly or separately?
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