The operation of the electrics makes or breaks a model. If it doesn't work properly then it will not give satisfaction and you will lose interest in even finishing it. Use paper and coloured pens and work out the polarity along each rail, where section breaks are needed and where points switch polarity, and even if using DCC it's still a good idea to set it all out and check it will work.
The biggest dilemma! DC or DCC?
Golden Rule 4 - Neatness.
When it comes to fitting the electrics be neat and tidy, clip up the wires and make the cable runs straight lines and clearly label everything.
Drill holes for power feeds (droppers) between sleepers as close to the rails as possible and solder wires to the outer sides of the rails neatly.
As part of the original planning, you should have decided on the standard to use, DC or DCC and also how you are going to control or isolate various sections of track if using DC.
You will need some form of control panel for point switches and any other accessory that requires a switch.
Try to keep consistent wire colour coding if possible and make the underside of the base boards as neat and interesting as the top sides, particularly for an exhibition layout.
People like to peer under and behind to see how it works. Check and double check your wiring, solder all joints and make sure it's robust before you bury any of it under scenery.
For a DC layout you can use a method known as cab control which is when more than one controller can operate it's own section or sections of track and any loco in that section, and all sections of the layout can be switched to any number of controllers. This allows one operator to drive a loco into a section then pass control to a second operator to continue. A variation on this sometimes called block section, is when only one controller is available but can feed power to any number of separate sections of track when only a single operator is in command, as in a home layout rather than an exhibition layout.
DCC is slightly different in that all track is powered constantly and you control each loco individually but the principle of keeping wiring neat and tidy is the same for both methods. You can control points and accessories in the same way as with analogue DC only there is no requirement for sectioning the power feeds, or adopt a completely digital route setting system using small computer modules which plug together and can be programmed to operate points, signals and other items.