The "safe" way to do it is to add the H2SO4 last. However, Piranha is never
really safe. It's extremely aggressive, much more so than straight concentrated
H2SO4. You should always be wearing a chemical smock; a face shield; and good,
thick chemical
resistant gloves when mixing and using piranha.

I've always added H2O2 last, myself. But, I have a flair for the dramatic.

Jesse Fowler

P.S.
For fun, you can put a small drop of H2SO4 on a clean wipe, and a small drop of
piranha on another part of the clean wipe. You'll notice that pure H2SO4 will
gradually disolve and discolor the clan wipe. Piranha will instantly turn it
black.