Design Documentation - Making a Map

Preface
Here are the most important and simplistic parts for making a reasonable map so you don't have to bother compiling all the resources yourself.

Brainstorming your world
Before we get into the science of it, there are a few things you will want to take into consideration that will affect your choices down the line. We're going to go downright basic here with things such as: The distribution or where you put the land combined with the weather will control where water is plentiful and poor - where are rain forests, were are the deserts, and so on.
 * 1) The distribution of the land masses and oceans, aka, The Aesthetics
 * 2) The severity of the weather, aka, how hot/cold/stormy/dry/wet you want it to get
 * 3) The overall topography - where is deep ocean, where are the mountains, where are larger volcanoes, etc

The overall topography plays a part in this too, but we'll go into that later.

Regarding plate tectonics –
You can wave away plate tectonics if you want to in a fantasy setting, but IRL these are dead and lifeless rocks. If you're trying to keep things in the realm of common understanding so that fantastical things seem more fantastical (since we have a common frame of reference) it's better to stick to using this stuff - and it's not that hard to do it on a basic level. Anyway! The shifting and reforming of continental and oceanic plates are due to the earth attempting to shed the heat from:

a)    The formation of the world (in earth’s case this heat is mostly gone), stored in the core

b)    Radioactive decay of elements in the mantle layer

This gets convoluted where stable subterranean worlds that exist wholly inside the planet such as the Underdark exist, but this can be mostly hand-waved by mapping out the Underdark before the surface world.

Planet-scale meteorology and biomes:
There are some things to keep in mind, again using Earth as a reference: These rules give us the six-banded circulatory system (N>S) as illustrated below (thank you NASA)
 * Heat rises
 * The equator gets hotter than higher latitudes
 * Warmer air holds more water
 * The planet rotates east to west

I Don’t Want to Know Why, Just Tell Me What It Means: 1.    At the equator the hot air rises up into the atmosphere and moves away from the center

2.    At 30 N or S (a third of the way to the poles) the air is done cooling and falls back down without much of the original water content.

3.    The wind drags itself across the surface, either pole-ward via the mid-latitude cell, or back to the equator

a)    Pole-ward: The wind picks up water along the way but dumps it when it touches the polar air, then goes back up to the atmosphere

So this means your world:

1.    Will be clear-skied and dry around 30N/S (and poles, maybe)

2.    Will be lush and fluffy where wind systems collide

In addition, please remember:

1.    The earth rotates. The Coriolis effect means that objects and air masses moving to the poles curve west, and for things heading to the equator we get the opposite, eastern tack.

2.    This follows that winds between 35 and 50 latitude will be mostly headed west, and winds between 0 and 15 will head mostly east

3.    These winds are surface-level for all intents and purposes which means they can be blocked/diverted/pushed up by mountains*

* rain shadows: when an air mass touches a Trump supporter on one side of a mountain and shudders all the rain away, leaving the other half of the mountain dry as mother nature’s crevasse (windward vs leeward sides)

Temperature and Terrain Features

 * Cold water doesn’t evaporate well, so even areas that don’t get a lot of water can still hold it well
 * Hotter areas can still be dry even if it gets a lot of rain (grasslands, for example)
 * Deserts skew towards the equatorial line a bit from 30 to get more sun exposure for that sweet, sweet evaporation.
 * Rain shadows in the north tend to cause drier plains and tundra rather than deserts, which are actually very rare
 * Coastlines make things damper! See the first bullet.

Mountains! Lakes! Forests!
 * Mountains catch rain with windward sides being flush with plant life in most scenarios. They’re also colder, due to altitude, which makes them hold onto water even better. Mountains in desert areas tend to host human and animal population explosions.
 * Tiny oceans that act like watering cans, sprinkling rain on anything further downwind.
 * You can use them to break up deserts, if they’re deep enough (the topography of many deserts could form a bunch of large lakes if it rained enough – ever heard of a “dust bowl”?)
 * If the lake freezes it will stop offering rain, but it’ll likely be cold enough at that point so that water isn’t evaporating anyway and the area wouldn’t need it to begin with.
 * Can act like lakes/oceans/rivers and offer up some moisture downwind as well. If you cut down that forest, you might see long-term consequences…

Just around the riverbend – aka, do rivers split?
Here’s a quick primer so that you’re not just dropping a heaping plate of spaghetti on your map, tracing it, and calling it good.
 * Water flows downhill, and goes for the steepest path of descent
 * Rivers start from rainfall
 * Rivers end in the sea or, rarely, evaporate into nothing

So how do I do this?

Pick heights for your terrain, aka topography, first. Doing this work backwards is the closest you might get to logistical hell so for the love of god just get a basic height map together in your head, if not on paper/pixel.

Water will, generally, flow along the same path – when rivers meet they will both flow in the same direction (ie no forking or splitting) so a good rule of thumb is that like adventuring parties, rivers should join up, not split up.

Because rivers travel via greatest descent, rivers don’t belong on the tops of hills or ridges. Raindrops tend to fall into “catchment” areas that are separated by hills/ridges that then collect the water into streams/rivers that form a drainage basin, and each basin will eventually narrow down to one river that will try to wind down to the sea (with the exception of a “closed sea”, which you can look at here)

To add to the above, this means you can have many “rivers” or feeder sources, but only one exit river.

The steeper the slope, the straighter the river – plains will have more meandering rivers that may change course over time as sediment builds up, whereas mountains are straight and fast

If Rivers Don’t Split, Explain the Nile Delta?

Rivers can pick up a bunch of sediment over time which it will eventually dump, mostly near the sea or a plain where it starts slowing down. This may cause the river to meander into a braid (collection of small branches of tiny rivers), which in turn will all tend to come back together again – but in the case of the Nile Delta it hits the sea first. If you really want to make a delta your river should be long enough to have collected the sediment for braiding – example, the Nile Delta is 100 miles N/S, whereas the river itself is 4000 miles long.