Porting tends to start by understanding both boards and the differences in pin usage and connectivity.
For the STM32 I tend to start with a simple donor project or template that provides minimal functionality, which usually consists of IO routines to a USART based terminal and a proper Hard Fault Handler which can print out useful diagnostics.
Keep a copy of this if you plan to repeat using the new target platform.
Then bring over sections of code you want to use, editing for differences and testing along the way. Have the IDE build along the way so you can fix errors quickly rather than deal with 100s later.
