Once you have entered the contact details for the lawyers and executors, making them a Key-Holder is very simple. You can see who are the existing Key Holders when you click on Contacts. There is a gold key displayed on their profile.

If you click on the profile of the lawyer or executor you will see a switch. By the side of it states "Enable this contact as a Key Holder". Just flick the switch to "on" and it turns gold or "off" and it turns red. Then click on the "x" at the top right, to close the view. You should then see the gold key displayed if you have switched it on.

You may have a maximum of 3 Key Holders.