Write one game. (Even if it's tiny, unpublished etc) Then write a different one, but take bits where you can from the last game. Any bits you reuse (or adapt) are now your engine! Ideally then go back to game0 and reapply the changed engine/common parts. Port it to another platform, renderer etc Tada! You have an engine :)

Repeat this over 20 years, rewrite from scratch every few years, switch to unity, unreal, then back to another rewrite!