In many cases, you don't have to do very much at all. In the Low Quality Posts queue, you can choose from a list of canned comments when recommending deletion. Like any other comments, these produce inbox notifications for the post author. If that user has the "email me my unread inbox notifications" preference turned on, they'll receive an email after some length of time that contains all the inbox messages they haven't read. Comments do not send immediate email notifications, but they are included in those general mail messages.
If the user doesn't have that preference enabled, and if they don't check back, it's unlikely that they're sufficiently invested in their post to care about the site's standards. In that case, you can't do anything, alas.
If nobody comments on the post before it's deleted, it's unlikely that the author will notice anything amiss. This is why I try to always leave comments with helpful information/links when flagging/voting for deletion.
If you're not a fan of any of the canned comments, writing your own is great! Excessive cheeriness and explicit welcomingness, though discouraged in questions and answers, make users feel much better and more likely to take your advice. Directing people to the tour and How to Answer is helpful. Tip: you can use
[answer] as shorthand for those links, respectively.
Relevant MSE: Notify answer OP when answer is deleted.