hope, proteus, tempus, elixir and goldballs.
the process is thus:
goldball creates unviable "eggs", one for each mutant they need to bring back.
the eggs are injected with the DNA of said mutant, thanks to mister sinister's genetic bank.
proteus warps reality to make the eggs viable.
elixer jump starts the life process and creates cell division.
tempus accelerates the eggs into husks that were the same age as the mutant when they died.
hope empowers the others and allows them to work in unison.
the memories and minds of the mutant are them implanted into the husk, thanks to Cerebro.
and boom. the mutant is brought back to life like nothing happened.