Marriage proposal should never be done in public, it's a very important decision which cannot be pressured into..

A proposal need to be in a private and non-pressurized environment no matter how confident you'll be getting the Yes word.

If you want it to be special yet on a budget.. here's a corny idea if you live alone and she drops by occasionally and got keys to your house.

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Estimate which day she'll drop by your house.

Have woman's high heel / shoes placed in front of your house porch

Have woman's lingerie [panties/bra] all over the living room and baiting to your room.

Have your room showered with roses across bed, Canon in D playing in the background, dim lights, champagne, on one knee with the ring wearing a suit with tie, leather shoes and everything.. looking as suave as you've ever been.

Just as she storms into the room to murder you, you'll go suprisee
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Note: If she enters the house and after 10min and hasn't enter your room.. most probably she commited suicide in the living room so please check after 10min or so..