Recommenders should have at least four weeks’ notice, and more is better, if you can give it.
This means asking, not announcing that you need a letter. When done well, these letters take a lot of effort. You do not need to apologize for asking for a letter of recommendation, but exhibiting professional courtesy is always important. Sending a note of thanks after you have submitted the application is also appropriate.
“Do you feel you can write a strong letter on my behalf?"
NCA encourages you to use this phrasing when asking for a recommendation letter. If someone does not think they can write strongly on your behalf, it is better that they tell you that up front so you can find another recommender.
Be clear about what you are asking of them
Make sure they know the deadline, how to submit the letter, and any formatting requirements (some scholarships have limits on word count). You may also need to provide information about the award (what it’s for, the selection criteria, etc.). If the scholarship provides you with a link to send your recommenders, share it. If NCA has a guide to writing recommendation letters for that award, send that, too. Put this information into your initial email.
Create a “highlights sheet” for them
If you have in mind some specific things that you are hoping they can write about, it does not hurt to create a short list of those items. A polished résumé could also do the trick. Did you write an exceptional paper for them? Send them a copy with their comments. Did you receive an award from their department? Remind them of that. And so on. Of course, they will have their own things they want to discuss in their letter, but it does not hurt to share supplemental materials—and to help them understand the role their letter will be playing in your portfolio.