等我喝完这杯咖啡就去做

等我喝完这杯咖啡就去做

“我喝完就去” “我喝完这杯咖啡就去开始做”

I would say: 我喝完这杯咖啡后再去写。 (I'll do it after I finish drinking this cup of coffee) ps: Chinese don't use 做 to describe writing a letter. In English, we can make a reference to an action by saying "do it". In Korean, we have the "하다". But in Chinese, we can't use "做" in that manner.

Since you have written the context, I would say： 我喝完这杯咖啡就去写信。