Open Facebook and go to your profile
Tap or click Friends
Select the pencil icon or More option
Choose Edit privacy
Set Who can see your friends list? to Only me
Save the changes
On mobile, open the Facebook app
Tap the menu icon
Go to Settings & privacy
Tap Settings
Scroll to Audience and visibility
Tap How people find and contact you
Tap Who can see your friends list?
Select Only me
Confirm the change
If the option is unavailable, use the desktop site or update the app