1. Modify <bugzilla>/localconfig to have the operating systems, severities, platforms,etc. that you want.
2. Modify <bugzilla>/template/en/default/bug/create/create.html.tmpl to have the look you want for creating bugs.
3. Modify <bugzilla>/template/en/default/bug/edit.html.tmpl to have the look you want for editing bugs.
4. Other files that could be considered for modification under the directory <bugzilla>/template/en/default/:
   • bug/show-multiple.html.tmpl – list bugs in long format
   • global/field-descs.none.tmpl – field descriptions
   • list/edit-multiple.html.tmpl – edit multiple bugs
   • list/table.html.tmpl – list bugs in short format
   • pages/fields.html.tmpl – description of fields
   • pages/bug-writing.html.tmpl – help page for writing bugs
   • search/form.html.tmpl – help page for searching for bugs
   • search/search-help.html.tmpl – help page for searching for bugs
5. Go to the <bugzilla> directory and run perl checksetup.pl.

To learn more, Template Customization.