You shouldn't need a 10-10-10 fertilizer. The only nutrient your tree should need if it’s mature is nitrogen (the first number) If it is growing in a lawn, you don’t want to add phosphorus (the second number).  Just an FYI - the nitrogen will cause your grass to grow. 

Application depends on the fertilizer you purchase. If it is liquid, mix it in the recommended strength and water it in all the way out to the drip line. Near the trunk is not so important. You want to feed the roots out nearer the drip line. If it is granular, sprinkle it according to package instructions and water it it (unless you’re expecting rain). 

https://extension.umn.edu/plant-diseases/apple-scab

https://extension.umn.edu/fruit/growing-apples#watering%2C-weeding%2C-fertilizing-and-staking-trees-263960

Be sure to clean up all fallen leaves and fruit as this will go a long way toward having a healthier tree next year.

We have seen a lot of Apple Scab this year.