Okay, so your school requires you to use the double space. How do you remember to do so?
You have 2 choices as I see it:
1. Try and remember to use the 2 spaces as you type.
2. Completely ignore the requirement and add it afterwards.
Trying to remember to do so as you type, places your focus elsewhere. If you are not used to using 2 spaces, trying to do so while writing an essay is hardly productive. Sure practice it, but not while writing. It will simply interupt your creative flow. Set aside separate time to just practice 2 spaces, if you want.
Do it afterwards. As I said before, a huge time waster. Use the Find and Replace function. But be warned, if you have any abbreviations, citations, or references, you cannot do one global change. You have to do it sentence by sentence. Have fun. As editors, this is literally what we have to do. Make sure to have the hide/show icon switched on so you can see how many spaces there are and if another is needed. Under 10 pages one remains sane. After that, one is seriously cursing APA and your school. But like many conventions, a simple enough academic hoop to jump through.