Its a while since I did this as an MDI diabetic, but I used to do it same way as I do now for pump stop my short acting insulin and fast for say around 6 hours, so morning, afternoon, evening + night. Choosing a different day for each with a few day gap between each fast. Then by comparing these you can see how well your basal dose is working without being impacted by food and bolus doses. You can then adjust as needed. Your diabetes nurse/doc should be able to have this laid out for you.