By formula, are you looking for a do-it-yourself solution? If so, I would advise against that, as it is much more likely to burn the leaves on your tree than a commercial product that is highly refined and broadly tested. Choose a commercial product labeled for control of aphids in apple (or fruit) trees. Spray when the temperatures are below 90 F, the tree is not under stress from heat, drought, etc., and it is not overly humid. Follow all label directions, including the timing for reapplication to prevent a resurgence. 

For more information:

https://www.canr.msu.edu/news/how-to-control-invasive-pests-while-protecting-pollinators-and-other-beneficial-insects