Make sure her hair and nails have been done recently, if not that day!
Try to coordinate with one of her friends and get them to have a "Girl's Day" either during the week or the morning of. The engagement ring is going to be a big part of not only the pictures the day of, but her showing it off for days after.
If you and your potential fiance are big and extravagant, be big and extravagant! If you're reserved, maybe choose something less crowded. She will react better when it's something that fits the dynamic of the relationship you have built.
Plan it out
Utilize Task apps and Calendars. Be as meticulous as you can when planning out a day. Research the area that you want to ask at. Talk to your photographer to see if they have suggestions. Odds are, they'll have time preferences.
Get some help
Try to get some friends help you with proposal ideas. Delegate some tasks. The proposal should almost be like a mini-wedding day where you both don't have to do anything but show up.
Careful who you put in the loop
Be mindful of sharing your plans with small children or even chatty adults. Even if they don't say things directly, they could still possibly give it away through behavior.
Don't be weird
This one may be the most difficult of them all to deal with! What I mean here is, try not to do extremely off-the-cuff things in the days and hours leading up to the proposal. You're going to be nervous, but you have to remain cool and collected.
Speak from the heart
100% of the proposals I have shot, the guy says "What am I going to say?!". The best proposal tip is to take a big breath to clear your mind when you see her and just start with her name. From there, instincts will kick in. Even if you're writing something before hand, this technique should prove helpful. If you're doing it right, she'll be crying anyway!