To add new decay mode, go to toolbox tab, then

1. specify where to insert the new branching ratio by clicking on one of grey bars between decay modes on left panel
2. drag and drop particles from 'particle selector' into the decay field. to find particles in selector - use Search field, or expand the section of needed particle
3. click on particle on decay field to remove it, if needed

4. Use 'advanced edit' link to edit decay text

5. Click 'verify and render' button to check decay syntax and see the preview. After clicking on 'verify and render' it will become possible to save new decay with 'create decay mode'' button

NOTE: While using edit text field instead of drag and drop, it might be useful to see unparsed texts of other decays as examples. to do so, click on decay modes on left panel, this will open their texts in popup window