Add notes in an Opportunity

1. Go into the respective opportunity.
2. Click on Notes in opportunity-menu bar as shown in the image.
   
3. Click on button
4. Write your notes. Use if you want to attach a file(image, pdf, excel).
5. To mention/tag other team members in the project, use symbol '@' this will pop up a drop down with opportunity team members > select the person you want to tag. 
6. Click Add to save it.