If you haven't heard about it, there is an important event scheduled for May 22 & 23, 2020 to do mass porting of Drupal 8 modules and themes to Drupal 9.
The following a set of instructions will show you how to contribute to that effort.
For a more detailed detailed account please see Kristen Pol's blog post, Preparing yourself for Drupal 9 porting weekend.
- First thing to do, of course, is to install Drupal 8 - the recommended way is to use composer. For alternative setups, see @mradcliffe comment below.