Write an outline for your story. This can easily be done on the computer but can also be written in a notebook if you feel more comfortable that way. Include in your outline a clear beginning, middle and end to your story as well as a clear description of the character you wish to tell the story.
Who is the author of this diary? When creating the character, keep in mind that everything in the story will be told through his eyes, so you will want him to be a strong, fleshed-out character. Make a list of the likes and dislikes for the character, and then jot down a couple of phrases he might say a lot. Once you've finished with your outline, set it aside.
Pick a date to begin your story. If you are writing a period piece, make sure the dates are accurate. D-Day wasn't on February 8th, 1950. If you use false information in your story, you have a chance of losing your audience.
Write the first diary entry. Think of the first entry as a speed date. We want to get to know your character but only just slightly. The subsequent diary entries will help us learn more about him as the novel progresses. It isn't important that the main plot of the story begin on the very first page, though it could if you wanted it to. Make sure you use a tone that is appropriate to your character and one that you can carry throughout the diary.
Go through the story, writing down the important events for the plot along the way. Think of it as if you were writing your own diary. Important things don't happen to you every day, and there isn't just one thing happening to you on any given day. You can write about multiple things, maybe your character gets a promotion at work and goes on a really great date. While the great date is what leads to the love story you are trying to write, you should still include the promotion, too.
You can also skip days. You can have one entry that is on March 20th, and the next entry might not be until April 1st.
All diary entries do not need be the same length. There may be a day where all you write is, "Work sucked. Tired." It gets the point across and keeps the reader connected to your character. It also helps keep the story more realistic to your readers.
Set the story aside for a couple of weeks after you are finished. Come back to it with a fresh set of eyes and read through it again. As you read through it, fix any grammatical errors you find and cut out any unnecessary words. You may also want to add some things in that will set up a later diary entry. Once you've done this, repeat this step at least one more time.