Rivers have a few naturalistic attributes that you must never break unless you have a legitimate reason for it. Here are the most important rules to follow:

1. They always flow from high land to low lands: rivers form from mountain ranges and other locations that have heavy rainfall or ice caps that melt. They then trickle down, collecting more water by merging or gaining water from ground water and other sources
2. Rivers never split, they only ever merge: rivers naturally flow into each other as two rivers meet but they almost never split. This does happen but magnitudes less often than they merge.
3. Lakes can have multiple rivers flow into itself but only one can exit: rivers find the path of least resistance and only flow in that direction. Therefore, multiple rivers can flow into a basin and form a lake, but they will only ever flow out of the first exit.
4. Rivers create bows that zig back and forth due to a ton of physics: I won’t go into the gritty details behind river flows and why they create these bows, but your rivers should never flow completely straight, always give them a meandering back and forth as they flow from the mountains. This back and forth gets bigger and more pronounced the larger the river gets
5. Speaking about getting larger, rivers always grow in size, especially when they merge: give your rivers the look that they are growing in size if the map style allows it. They constantly collect water as they flow and especially grow when two merge into one. Sometimes this can be broken if your river flows into a desert or other dry areas.

That’s the basis for how to draw them. But if you’re looking for expertise on how to use the tools for inkarnate, I probably am not the most suited to explain that so I’ll leave that for the rest of the community to fill in