1. ask her parents
while it may seem like an outdated tradition, it's an important one to prioritize and value // asking them for their blessing shows you care about and respect their approval // it gives them an opportunity to voice joys and concerns and gives you an invitation to conversation, allowing you to communicate how you'll care for their daughter // in the end will gain you major points as a future son-in-law.
2. tell your parents
obviously....right? not always // it's easy when we've been on our own for awhile to get caught up in our own lives and our own plans to forget to mention it to the parentals, but put this on priority as well out of respect for them and family in general // they raised you and should be at the top of your list
3. decide who you want to invite
whether it's an intimate moment between the 2 of you or a "family reunion" type of proposal, decide who you want there to celebrate once she says yes // if you are inviting her parents or siblings be sure you are the one inviting them; don't ask her to bring everyone // that's a dead giveaway and yes that may seem obvious, but i've seen it happen before and the surprise is no longer a surprise // so, be sure when you decide who you want there, you do all of the inviting and keep it as secretive as possible.
4. create a backup plan just as good as your original plan
i cannot stress enough how important your backup plan is // you may believe everything will go as planned (and i hope it does), but more often than not, plans get delayed, rearranged, or even cancelled // prepping for that to happen will only benefit you // without one, panic will set in and your stressed behavior will allow your girlfriend to put 2 and 2 together // so do yourself a favor and make a backup in the event the original doesn't go as planned.
5. check the weather
if you're thinking about proposing anywhere outdoors, be checking the weather often, especially as the day gets closer // it's an easy detail to forget in the midst of planning, but be sure it's on your radar // if it's not looking good, you can easily transition to your backup plan without breaking a sweat.