Salesforce Apex Code for Preventing Changes to Attachments on Opportunities

General information for the business: Want some code and instructions for salesforce internal controls
Kind of development: Customization of existing program
Platform: Salesforce
Num. of modules: 1
Description of every module: I'd like some code (I'm told it's apex) to prevent any deletions of files attached to Opportunities when a particular Opportunity Custom Field (checkbox) is checked.
Description of requirements/functionality: I need instructions on where to put the code.
Extra notes: You won't have access to our salesforce instance