After the 'Preparation' step of the installation, SUSE will bring you the 'Installation' step itself.
Select the tab 'Expert'
Then click 'Change...' and then 'Partitioning...'.
Choose 'Create Custom Partition Setup' and then click 'Next'.
Choose 'Custom Partitioning' and click 'Next'.
There you'll be able to Create, Edit, Delete and Resize partitions.
Maybe starting resizing the Windows one
Again, it won't be easy to follow and help you further. You'll need time and reading to do so.
Probably you'll mess, be wrong, etc. I see no other way to learning