My M has this problem. It's not easy cutting all the metal away and fabricating the new shapes. I do it outside, so I can only work on decent days. Luckily, the damage is covered by the outer sills, so the repair doesn't have to be as neat as something that you will see every day.
It takes me about two days per jacking point, but two days of labour may take a month to find. Two hours here, an hour there, etc.
Lots of hammering to confirm what's strong enough. Lots of sweeping up the rust that falls out of the car. Lots of flap disking and cleaning to prepare the car. Lots of painting to protect what's left. Lots of cereal boxes to get the shapes right. Lots of bending of thick 1.5mm steel (too thick to bend by hand). Lots of trial fitting. Lots of welding. Lots of flap disking to get things looking presentable, and to make the outer sills fit back on.
It will never be the same again, as the jacking point plasic blocks now attach with bolts into captive nuts, and the securing bolts will self tap into the new steel. Even with lots of copper grease, I still expect them to be rusted solid within 12 months.
It's also a temporary thing, as welding heats the steel and changes the properties. It may last a decade, but next time the repair will have to be even bigger.