Open Facebook and go to your profile
Tap or click Friends
Tap or click the three dots or Edit Privacy
Select Edit Privacy
Change Who can see your friends list?
Set it to Only me
Save the changes
On mobile, you can also go to Settings and privacy
Open Settings
Go to Audience and visibility
Tap How people find and contact you
Adjust friends list visibility if available
Check your profile from another account to confirm the friends list is hidden