You should always have an alternative plan of action in mind, should your flight not proceed as planned. If you fly long enough, things will not go as planned at some point. usually, it comes down to weather being worse than forecast. A system malfunction, sick passenger, whatever it may be, it may be a better decision to divert to another airport, rather than continue to your original destination.
Under VFR, diversion is simply a matter of doing the best you can to do some quick flight planning while already enroute. The most important aspect of this enroute planning is to make sure flying the airplane is always the number one priority. It may seem silly to say, but, it is actually easy to forget to fly the airplane first when distracted by other tasks.
To divert, plot a course, choose pilotage checkpoints, measure distances, calculate groundspeed with your flight computer, and make dead reckoning estimations of time enroute and arrival time. Check to make sure you have enough fuel to arrive and the new destination with applicable fuel reserves. Do these things according to your ability. If you are unable to fly the airplane and do everything, flying the airplane still comes first. For example, if the diversion airport is close by, you may not need to actually plot a route. Make the best and most accurate planning you are able to, while still flying the airplane and visually scanning for other traffic.
One way of making a diversion simpler is to, all other things equal, choose an airport that has a VOR or NDB on the field or very nearby. Navigation to an airport with a VOR or NDB on the field is as simple as flying to that NAVAID.
The main goal is to maintain good situational awareness, to know where you are, how much fuel is on board, and how much fuel you will have when you land. Note the time you begin the diversion from your original course. Your plan can be simple, it just has to exist. You shouldn't ever be in a position where you are thinking things like "I should have enough fuel to make it there" or "I think that will work". Decide a new plan of action, and make it happen, while maintaining good situational awareness. If your new plan of action is not working, you are free to change it again. But, be in control and command the situation.
Emergencies
Any emergency situation takes priority over everything except flying the airplane. Handle any emergency situation prior to attempting to perform navigation tasks. In an emergency situation, plotting a course or making estimations is very likely not practical. Fly the airplane, handle the emergency, visually scan for traffic.
Any way you can reduce your workload while flying helps you to fly more safely. A simple, easy to follow diversion can be followed, even during an emergency. A pilot should be able to handle an emergency while maintaining situational awareness and staying on course to an alternate airport. The pilot is able to do this by using resources to make maintaining situational awareness and navigation to the alternate very easy, freeing them to handle whatever problem exists.