Bag? that would suggest granules rather than a bottle of the stuff. So far as I'm aware, the granules should be dissolved in a certain amount of water, usually one sachet per gallon of water, then applied, but it depends which manufacturer's version you've bought. Contact the manufacturer if its not clear on the pack.

in the past I have sprinkled the granules liberally around the plant [camellia in my case] and just watered in.

My reading indicates that they need a soil pH similar to roses, so I would follow those directions, to start.

Now I think about it, most Skimmia do fine in ordinary soil, they don't need a highly acid one - the exception is Skimmia reevesiana, which isn't keen on limey soils.

It won't do too much harm to use some granules around it, but perhaps your Skimmia gets too much sun - that can make them look chlorotic and yellowish. Most prefer partial shade conditions or dappled sunlight.