what was dumbledore doing to get a scar in the perfect shape of the london underground?

rebuilding London underground to perfectly match his scar

Branding himself with the maps of all the public transport routes in London overlayed on one another, but the first one hurt way worse than expected so he stopped with the just the underground

For fun, he tried to make sherbet lemons the muggle way, and failed spectacularly.

He made some bad choices before he got into wizard school

I suspect it was something to do with Grindelwald and their "adventures". Or it might be Grindelwald and the fight that got baby Sis killed.

He tripped over McGonagall in her cat form once.