So I am dating Margaret and she is at 14 love points so I want to marry her: I have the double bed but I don't know how to make an engagement ring. Oh and my crafting level is 7 and I am dating Margaret.
Use an Ore and a Gem, even Iron and Amethyst work too. It's in the accessories section, at Lv20. You don't need to receive the recipe for it to work either. If you end up marrying someone before you get Eliza's request to make one, she'll even comment on it.
According to this, you don't even need a double bed either. All bachelor events end in reverse proposal, so just accept and you're good to go! Of course, you can always turn them down; but then you'll need the ring on hand if you decide to go with them later on.