To help your soil become acidic enough for Hydrangeas, you will need to alter the soil quite deeply. If you don’t go low enough, they will potentially be drawing water from limey soil instead.
This could mean altering your soil at least 30 cm/1 foot deep. It will be quite a lot of work, but worth it for the impact of blue flowers!
Firstly, you will want to dig in a considerable amount of ericaceous compost. This is designed to be acidic for acid-loving plants like heathers, azaleas, and rhododendrons. The name ericaceous even comes from the Latin ‘Erica’, meaning ‘heather’!
I would suggest that you use some protective equipment before moving on to the Sulphur application. Gloves, goggles, and a dust mask should provide adequate protection.
Then, apply your Sulphur. You can sprinkle it over the surface and water it into the soil, but this may take a while to penetrate. If you can, cultivate it into the ground with a rotovator.