I found the solutions above way too slow. I have all my sagemath notebooks on a sagemathNB folder in Documents.
In my ~/bin I created a script called sage_nb
#!/bin/bash
cd ~/Documents/sagemathNB
$SAGE ROOT/sage -n jupyter
Remember to chmod +x sage_nb
Then I create a SageMath.desktop file in my desktop
[Desktop Entry]
Name=Jupyter notebook with Sage
Name[en]=Jupyter notebook with Sage
Comment=Scientific Computing using Jupyter notebook and Sage
Comment[en]=Scientific Computing using Jupyter notebook and Sage
Exec=sage_nb
Icon=$SAGE ROOT/src/ext/notebook-ipython/logo.svg
Terminal=false
Type=Application
Categories=Education;Math;Science;
StartupNotify=true
Name[en_US]=SageMath
Finally you can copy to sudo cp SageMath.desktop /usr/share/applications/