Big leaf hydrangeas, identified by their broad oval-shaped leaves and large flowers, are called macrophylla hydrangea shrubs.
They bloom on old wood so should only be pruned after they have bloomed and once the blooms have faded. This is so the new buds have time to form in their new branches so that they can bloom the following year. Never, ever cut them right back to the ground. Just selectively prune out any dead branches after the blooms fade.
Note whether your shrub’s blooms are white or colored. This is important because the soil makes a difference if they are blue, red, or pink. For example, if the PH is below 6, your soil is acidic and the flowers will be blue. If the PH is above 7, the soil is alkaline and the flowers will be pink or red.
You can grow the shrubs in large pots and control the soil you use or you can try to amend the soil you have by adding lime, e.g. a high phosphorus fertilizer. If you use minerals, dilute them well and add sparingly to the soil according to label instructions to avoid scorching. Sometimes soil can be just slightly acid or neutral with a PH of 6 or 7 and you will get a mix of both pink and blue flowers on the same plant.
An Extension Agent can test the soil before you plant to ascertain your PH level in advance. However, many of us inherit the shrubs in our gardens and have to observe them to understand what they are and what soil conditions are present. If your soil is extremely alkaline, it is pointless to try to change it. Just stick to hydrangeas that are white or pink or red.