A basic recipe, you can see that all the parts are at a 2:1:1 ratio by volume, so multiply and scale as needed. The syrup is excellent with drinks, as you would traditionally use a shrub.
Yield: a little over 1 cup
Stir the vinegar and sugar until melted, then add the milkweed flowers and allow to sit in a container like a mason jar, with lid, at room temperature. Strain after a week and store in the fridge for the best flavor.
Notes
Shrubs are wonderful for making drinks. I love adding this to Non-alcoholic cocktails. To make one, mix the shrub with half soda water, mix, and pour over ice. Feel free to adjust the ratios to your taste. Muddling in some fresh herbs like mint or basil is good too.