I realise that there can be as many answers to this question as there are different circumstances. So, if your opinion differs from mine, that’s OK! (You’re welcome to let me know what you are finding most trying.)
In my experience, however, I’d emphasise two danger areas: money and parents.
Budget
When planning a ceremony, you have to be singing from the same hymn-sheet. Decide how much money will be available. Talk this over with the relevant people and do your best to reach a compromise, if you can’t agree on everything.
An example of an issue that needs sorting may be how much religion (if any) you want in your ceremony. Can you manage some give-and-take, if you’re not in full agreement? So, could the ceremony be mostly secular (but spiritual) with a conventional prayer or two included?
Or, depending on the ceremony, could you make it secular, but with someone doing a (religious) blessing?
There’s also ritual – you could borrow from various traditions, if you wanted. Some are “heavier” than others, so you could find one (or more) to suit.
These are things your celebrant could helpfully discuss with you.
My final point about expenditure is that, although you should not exceed your budget, you may find something you want that costs a little more than you planned for. It may be possible to accommodate this by juggling your costs. For example, you could possibly afford the celebrant of your dreams (me, obviously!), if you picked a weekday for your ceremony (often cheaper) or chose flowers that are in season …
Parents
If your parents are bankrolling your event, or they have strong personalities and feelings about, say, religion, you will have to take their wishes into consideration. Again, you may be able to compromise a bit. Think it through before you draw up battle-lines! What can you afford to sacrifice (or include) for the sake of harmony?
The bottom line of the matter is that it is YOUR event and you should be able to have it the way you want. Sure, there may have to be a little that you’d rather not include, but your parents must accept that their role is to enable you to have YOUR day. You should be grateful and, where reasonable, amenable, but you want to end up with a day that is memorable for the right reasons!