After logging in, click on File a Bug.
Choose the respective product about which you want to report a bug.
Next, choose the respective component. Below I have chosen Router UI UX.
Write a short summary of the bug/change.
Then write a detailed description explaining the bug/change.
Scroll down and click Add Attachment. Write a brief descripton of the attachment.
Finally, hit Submit Bug.
You will get a message that your bug was created. The email will be sent to the respective person who is responsible for solving the issue.