Your CO's endorsement must be completed before you can undergo your interview because the members of the board which interviews you must have it (along with other documents) before the interview. (The interview board's report has to be included in your completed application to CGRC.)
Many commands have their own ideas as to what the endorsement to your narrative memo should look like. Unfortunately, most of them are incorrect. The guide to creating endorsements is the Correspondence Manual (COMDTINST M5216.4C).
The endorsement is not to be a stand-alone memo from your CO to the Coast Guard Recruiting Command.
According to Art. 4.B.1.b. of the Recruiting Manual, "The CO must be a commissioned officer; an endorsement by an officer-in-charge is not sufficient." (emphasis in original)
The CO should not merely sign or initial the Thru line (i.e., endorse your memo without comment). Even though approval of your memo is implied by such an endorsement, it will indicate to the members of the boards who read it that your command doesn't think enough of you to say anything good about you. That is, a signature endorsement will likely be taken, at the very best, as a lukewarm endorsement.
The endorsement should not comment on your abilities in your rate. It's not meant to be a rehash of the comments section of your most recent marks. Everyone who applies for an officer program is going to have extremely high marks, so that info tells no one anything new. The interview and selection boards want to know if you have the traits they think an officer should have, not how well you've done as a petty officer. The only rate-related information that should be included in the endorsement is that which relates to officer duties and qualities, and your potential as an officer.
The endorsement is meant to be a concise statement of what your command thinks of you and whether you will be a good officer. Such an endorsement lays out what your CO thinks are qualities an officer should possess and states how you have or lack those qualities.
There are several specific things the Recruiting Manual requires your endorsement to specifically address. These are:
an evaluation of your potential value to the Coast Guard as a commissioned officer;
an evaluation of you compared with other personnel your CO has known who have completed the program you're applying to;
any outstanding professional or other qualifications you may possess;
a statement expressing knowledge of your ability and willingness to meet your financial obligations;
your average marks during your current enlistment according to Article 1.B.31 of the Military Separations Manual (COMDTINST 1000.4), and your most recent marks for Performance, Leadership, Military, and Professional Qualities Factor; and
a statement certifying that the command conducted a review of your record and that you meet character standards identified in Article 4.B.1.a. of the Recruiting Manual.
The following paragraph, at the end of the endorsement, should satisfy this last requirement:
I certify that Petty Officer Washington meets all qualifications for the _____ program as set forth in the Personnel Manual (COMDTINST 1000.6A), the Recruiting Manual (COMDTINST M1100.2D), and all other relevant directives and ALCOASTs.
To enable your CO to certify you meet the standards, s/he may want your SPO or Yeoman to check your PDR against the qualifications for the program you’re applying for and to advise the person who’s drafting the endorsement accordingly. You may find it to your benefit to provide the SPO or Yeoman with the qualifications for the program you’re applying for copied from the relevant manual(s) so they don’t have to find them, if that person isn’t familiar with them.
You're not required to provide anything besides your narrative memo to those who draft the endorsement. The assumption underlying an endorsement is that the person drafting it knows you well. If the drafter doesn't, an interview may be in order. But the verbiage in the endorsement should be based on personal knowledge of you rather than documents.
Those who draft the endorsement don't need to see your completed application folder. In fact, the Privacy Act and the Health Insurance Portability and Accountability Act (HIPAA) prohibit most people from seeing much of what's in your application folder unless they have a reason stated in the law. And, as a practical matter, there's little in your application that will help them draft the endorsement.
In addition to your CO's endorsement, you can also include two letters of endorsement (only one of which may be from someone in the Coast Guard). Before requesting letters of recommendation, you should read "How to Get a Great Letter of Recommendation".