A SIMPLE KEY FOR $1400 STIMILUS CHECK UNVEILED

My wife or husband & I will both of those be turning 70 in 2024, so We are going to start taking social security. I recognize that 85% will likely be taxable & that we will specify nearly 22% be withheld for federal tax. So how Is that this performed? Is barely 85% on the month-to-month check taxed at 22% or is the entire every month amount taxed?