We built a detached garage this year which is used for our home business. What would be the best way to get deductions for this addition?

You will enter the detached garage as an Asset for your business so that you can depreciate the cost.

You do this as you are entering the income and expenses for your business.  While you are in the business section of the return, there is a separate category for Business Assets where you will enter the details for your detached garage.