There are many ways to pop the most important question in your life. It will be more difficult if you have to propose in Japanese.

You can make it short by saying 結婚してください, meaning 'please marry me'. The pronunciation is 'ke kon shi te ku da sai'

You can also say あなたを一生かけて幸せにします, meaning 'I will make you happy for ever'. The pronunciation is 'anata wo isho kakete shiawasei ni shi masu'.

Good luck!