You need to start with data...
First the internet is a wealth of information even though there is a whole lot of noise amoung the data. For example:
https://www.fleet.ford.com/truckbbas/non-html/2003/bb_pdf/204-205.pdf
In particular you want/need to know how much current the various wire gauges can carry. This will also vary by temperature, wire bundling, and wire length. I have a couple of other sites at home that provide similiar information, but you know how internet searches are: you never get the same stuff twice.
Every accessory has a current draw. This is a critical number as it determines how big the wire feeding it needs to be and what size of fuse to put in the circuit. This will also drive the breakdown of circuits to some extent. For example a logical circuit might be "lights" except that the headlights pull enough current to be a circuit all their own. Note that the current draws are often not published
So you will have to reverse engineer them by reference to existing wiring diagrams. They all were used in some vehicle, get wiring diagrams for that vehicle and extract circuits, wire sizes, and fuse sizes to get some idea how you should set them up. This will be much easier if the accessories were actually used on some variation of the same car. In this case you may already have some wiring and such present to support the accessories. For example power window wires may already be there and if not, at least a fuse block position for the windows.
Note that in general the fuse should always be rated lower than all of the wiring connected downstream. You always want the fuse to blow rather than the wire. Fuses don't emit smoke and we all know that when the smoke gets out of electronic stuff, very bad things happen.
Once you have the basic circuit and wire/fuse sizes figured out then you can figure out how to add them. Part of this is deciding if this is an always on circuit or only on with ignition or with accessory. If the accessory draws significant power you will want to utilize a relay to drive power to it's circuit so as to reduce loading on the existing accessory or ignition circuits (they will drive the relay at low current and the relay will switch the higher power for the add-on circuit). To figure out if a relay is needed you will have to also figure out how much current capacity you have in the existing circuit that you want to add on to.
Because different car models may have already had some wiring infrastructure in place, there are a lot of variations to the add-on puzzle. Presuming a complete and total add-on, you can either add an auxiliary fuse block or a new wiring harness with a larger fuse block. Painless Performance Products has both types of setups available:
www.painlesswiring.com
Although I have not read them, Painless also has a couple of books that may be helpful as well.
Geez... feels like I've written a novel and I have not even gotten into wire routing or connections or the wonders of loom tubing or...
Steve