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The ARIZONA GRAVEL COMPANY is a good example to show how a small layout is wired. More can be seen about this small layout by clicking on the link above. The track plan is basically a passing loop with a spur in each direction. However each of these spurs is slightly different, as one comes off the outside of the loop, while the other comes off the inside of the loop, and will have to be wired differently.

Because this article is on how I would wire this layout, I will be allowing for slow motion turnout motors and live frogs in the turnouts, with the turnout controls on a small panel to be located on the front of the layout.

When wiring a layout the first and most important thing to work out is where the power is going to be feed to the tracks, where the power will run and where conflicts can occur to cause short circuits. These conflicts normally occur at the turnouts and this is where cuts in the track have to be planned. A simple drawing will show us where potential problems will occur. The main power feeds to the tracks are shown as red and blue dots on the diagrams.
Because we need to feed positive power into one rail and negative into the other so we can run our trains, anywhere these two meet will cause a short circuit. It's best to stop this happening as you wire the track and put cuts in the rails. I do this by doing simple drawings as can be seen at left. I first draw all the rails in the plan with a pencil. With a black pen I mark the 'X' where the frogs are, as this is where 99% of all short circuits occur. Next, using a red and a blue pen I mark with a large dot where the power is feed into the track. With a passing loop, power is feed in either side of the loop. Then I mark the rails with the red and blue pens from the feed points up to the black frog as in the diagram.
We now need to look at each turnout and work out where the power from each frog will run to. Let's start at turnout 'A', and  mark the rails with a green pen as in the diagram at left. The frog will have to be powered through a micro-switch so when it is thrown to the straight mainline the frog area will be blue power, and when it is thrown to the curve it will have red power. This can be seen in diagram 2
Next turnout to look at is the one at 'B'. Because the train passes through turnout 'A' to get to 'B', they should be linked so the siding at 'B' will only get the right (blue) power if turnout 'A' is set to the straight, as shown in diagram 3. 
If turnout 'A' is turned to the curve, the micro-switch  will turn the frog to the red power, as can be seen in diagram 4. This makes all the rails in the straight main and the spur a 'B' have the same type of power. Locomotives would not be able to run, so they can isolated there.
The same can be done for turnouts 'C' and 'D', with each turnout having it's own micro-switch to route the power for the frog. With the turnouts set this way, a locomotive will travel through bottom passing loop.
Wiring is always a difficult subject to describe. Above is only one example, and the only way to master wiring a layout is to do it. But the most important thing to do is check the wiring as you go, and fix any faults as you go. Do not do all the wiring and then check it. It's always harder, more time consuming and really frustrating that way.