Letters of recommendations is definitely one of your most important parts of your application. You definitely want to thoughtfully think of who you would want to choose to write your letter of recommendation. This is also not something you want to wait last minute on. Think through all of the people who would be willing to write your letter and then from there choose which schools you want the person to write for.
I definitely recommend starting to think of who you want the people to be to write your letters of recommendation by the beginning of the Summer you plan on applying that Fall. The reason why you want to start early with this because by the end the summer or beginning of the Fall you want to prepare emails to all of the people you want to write your letter. It's always good to give them a lot of time in advance to they are not scrambling at the last minute to write your letter. They will also respect this a lot because you are giving them a couple months ahead of time for them to write your letter.
Thinking of who you should select to write your letters of recommendations can be a difficult decision. Here are some tips when it comes to thinking of the right people to write your letter.
Now you should have a good idea on who to ask to write your letters of recommendations. The last step is now contacting them and figuring out what to say. GradStart provides an email template you can use here. This email template outlines all of initial information your recommenders need. Since you might be asking people who interned for or did research for a couple of years ago its important to establish the relationship you have with them within the initial email.