Remove the Starter You will have to remove the starter from the vehicle
Once you have it off you need to take the end cover off the solenoid, its best to use a 8mm wrench or socket.
Old plunger Once the cover is off you can take out the plunger and spring. The plunger will look like this
You can then see inside the solenoid to the starter solenoid contacts.
Compare contacts Its the wear on these, combined with the burned surface on the plunger that is causing those clicks when you go to start the engine.
Keep parts in order Make sure you keep the pieces in order. Do not lose any pieces and make sure that they fit back in OK. Some of these pieces will break.
Styles of Contacts There are several styles of contacts make sure you use the right ones.
Old and new contacts The starter solenoid contacts shown are both new (bright) and old (dark). If you look closely you will see on the old one there is an area worn away on the flat surface that contacts the plunger. Its this extra distance that the plunger need to travel that caused the intermittent connection and the clicks.
Clean up the plunger Clean up the plunger as shown. Use light grade sandpaper
Replacing the plunger If the old plunger does not clean up very well or the contact area is to thin, replace it. Dont forget to put the spring on the new one.
You need to take off the starter solenoid contacts from the large bolts on the starter case and "tap off the old starter solenoid contacts" from the bolts. You install the new starter solenoid contacts the same way. You will see that there are some insulators protecting bolts from the metal case. Its important not to forget to reinstall these the way they came off. When finishing the insulation of the contacts make sure that they are sitting flat in the starter. If they are not, (and in tightening the lock bolts they can twist) the starter will click again after a couple of days.