To process a share buy back (i.e. selling shares back to the company for a particular shareholder) in Cake,
Head to Equity from the main menu on the left;
Go to Shareholders;
In the top right corner, click Actions;
Choose Buy back or Repurchase depending on your region from the dropdown menu;
Search for the relevant Shareholder, Share Type, enter the Date of the Buy back, Number of Shares being repurchased by the company, and the amount paid and then finalise by clicking Buy back or Repurchase.
[AU Only] Remember that this does not process it with ASIC so you will also need to submit the relevant forms to them.