You can create multiple types of "patches" with Fossil. Unlike the common convention in Git, the first two examples here uses uncommitted changes to create a patch (although you could very well create patches of committed changes). The preferred method is by creating a plaintext patch by doing the following:
fossil diff -i > your-changes.patch
You can also create a binary patch:
fossil patch create your-changes.db
If your patchset is complex, and needs to be splitted in multiple check-ins, you can create a Fossil bundle:
fossil bundle create --from CHECKIN --to CHECKIN2 patchset.bundle
After creating the patches, you can simply send them to the mailing list, or upload the patches to the Fossil forum of the relevant repository.