First you have to find an owner of a B747 that is prepared to let you fly it and then, big then, you have to find an underwriter who will insure you against third party risk and legal liability, millions, possibly billions of $ worth of risk, (that assumes the owner is going to self insure the hull), no self respecting underwriter will insure you if you do not have relevant training and experience.
The pilots that fly big aircraft on PPls will, almost certainly, have several thousand hours of relevant experience on other similar types or the same type in the military.