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Here is a link to an article with a video that explains the process of how this is done. It is certainly something you can do yourself if you are up for a challenge!

http://mikesbackyardnursery.com/2014/04/how-to-grow-train-flowering-hydrangea-shrubs-into-tree-hydrangeas/

You will need to read the label and check the N-P-K number on the packaging.
The link below will refresh you on the fertilizer requirements of Hydrangea.

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/fertilizing-hydrangeas.htm

Your plants may have been burned by the fertilizer.

You an try washing the plant with a spray of water.
Fertilizer can burn the foliage and the roots. It can kill the plant.

You will need to wait and see how the plant progresses.

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/fertilizing-hydrangeas.htm

The old faded blossoms can be pruned off of your plant.

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/prune-hydrangea-bushes-hydrangea-pruning-instructions.htm

I think this could be mites or Aphids.
This could be treated with Neem Oil.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Adding good-quality compost is probably the best thing you can do for your Hydrangeas. This article may help you:

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm