This is a tough project for a do it yourself as the 5.4 liter motor on the Navigator is a dual over head cam and it's fuel injected. A car shop will charge you around $280 for the service but if you really want to attempt it yourself you will need a standard wratchet set with a swivel and a whole lot of patience. Remove the fuel rail and be careful not to drop any dirt in the injector. Remove the dust covers and begin replacing plugs one at a time. This requires a lot of lying on the engine and contorted positions but with enough patience it can be done by the average joe. I would spend the extra money and have a shop do it and spend my free time fishing.
Middle of valve covers ....take 7-8 mm and remove ...small cover to uncover plugs beneath wires/cap/coil pack. 5/8 socket
Follow the spark plug wires and have a tool ready to remove the spark plugs. They sell them at the aut parts store for cheap usually. Take off the cables and unscrew the old spark plugs. Then make sure the new ones are gapped at the right size and put them in and tighten them up. When changing spark plugs you usually want to change the wires also because they will usually go bad after and cause more problems then you have it made. If the spark plugs are hard to get to then you will need to have some one help you maybe and take it to a mechanic or take some things out and remove them and put them back in after. Some are hard to get to. It is cheaper if you can do it yourself. If you have a v6 depending on how many cylenders there are that is how many spark plugs you will have in your vehicle and have to change.