That's it. You do not need to use fancy legal wording. Your boss and your HR department know that sexual harassment is illegal and they should take it from there. You can expect that they won't just believe your side of the story from the start, as it's their obligation to approach this complaint from a neutral standpoint.

They will conduct an investigation. In some situations, such as a complaint about Bob looking at pornography in his open cubicle, your employer wouldn't need to tell Bob who complained. But, since this is a case in which he's accused of harassing a coworker directly, he'll know who made the complaint.

The law prohibits retaliation for making a complaint about sexual harassment, so your boss and the HR department shouldn't punish you in any way for making this complaint.

If your company doesn't take care of the issue, you can either file a complaint with the EEOC or hire your own employment lawyer to help with your case.

[Note that she mentions every incident, even the one that she doesn't have an exact date to note. The first incident is not actually sexual harassment—asking a coworker out on a date doesn't qualify. But, it shows when Bob's behavior started and how it quickly became inappropriate. Also, note that she gives the names of possible witnesses. It will help when they begin their investigation.]

Thank you so much for looking into this matter for me. I'm happy to give you any additional information as needed. I would very much like Bob to leave me alone so that I can do my work.

Sincerely,

Holly Jones.

On June 10, 2017, Bob followed me out to my car and berated me for not dating him. I believe Richard Thompson, from accounting, was also in the parking lot at the time, but I don't know if he overheard anything.
On June 13, 2017, Bob made a derogatory remark about me in a staff meeting. Steve, Karen, Jane, Chelsea, and Justin were there. Jane told Bob to knock it off.
On June 15, 2017, Bob sent me an email telling me that if I didn't go out with him, he'd tell Jane that I had plagiarized the marketing slides I created for the regional meeting.

The following incidents have occurred:

Approximately one month ago, Bob followed me to my car and asked me on a date. I said I wasn't interested and left. I did not think anything of it.
On June 1, 2017, Bob asked me out again. I told him no, I prefer to keep work and personal life separate. I told Steve and Karen the next day that Bob had asked me out.
On June 3, 2017, Bob sent me an email expressing his interest in dating me, and telling me I was “hot.” (The email is attached). I responded (also attached), “Do not contact me for non-business reasons.”
On June 9, 2017, Bob sent me a text message that included a photo of his genitalia. I deleted this.
On June 9, 2017, I asked Jane to reassign me to a different project because Bob was bothering me. She declined to reassign me, but I did not tell her about the email and the picture.

I am writing to notify you that Bob has been sexually harassing me. [Again, say this in a straightforward manner, right at the beginning.]

Subject Line: Official complaint of sexual harassment. [This is actually the most critical part of your complaint. Giving this as the subject line makes it impossible for anyone to say, "I didn't realize she was experiencing sexual harassment." They cannot, legally, ignore your complaint when you begin it that way.]

Dear Jane and Stacy, [By sending it both your boss, Jane, and the HR manager, Stacy, you're keeping everyone in the loop. There are certainly times you'll only want to send it to HR or only to your boss, but unless you have strong reasons to keep one of the two out of the loop, send it to both at once.]