First step is to check if there is already a porting branch (usually called `frameworks` or `kf5`). If not, you can create such a branch locally before posting it for review.

There are porting guides https://doc.qt.io/qt-5/portingguide.html and https://community.kde.org/Frameworks/Porting_Notes

It also helps to check how other repositories are ported; start with smaller ones to look for porting commits, sometimes grouped in the form `KClassName -> QClassName`.

Which repositories do you plan to help porting?