IMO, the firstmost source is your own observations. 3x cube is very tactile, so some moves are just natural.
It helps also to develop some sort of notation for yourself. This way you can track and repeat your moves.
Solving by layers is kinda logical. So solving one side (first layer) is not hard. Then some experimentation with rotation sequences which temporarily break the solved layer/face and then re-assemble it will lead to discovery of moves to swap the edges into the second layer.
The hardest then is to solve the third layer. Again, the notation and observations help charting your way through.
A curious discovery may be about some repeated pattern of moves which may be totally shuffling the cube yet, if continuing it, eventually returns the position to the beginning state. It's kind of a "period".
Solving by layers is logical, it's what most beginners learn, and it is kind of how CFOP (the most popular speedsolving method) works. Nevertheless, it's not what I would recommend. The problem with solving layer by layer is that you are sort of painting yourself into a corner from the beginning. After you have finished the first layer, you can't really do anything without breaking the first layer. Of course it is possible (and necessary) to proceed in a way where you keep breaking and repairing the first layer while progressing with the rest of the cube, but the limited freedom of movement still makes the solution process needlessly complicated, and increases the move count.
In my opinion, it's better to start by solving a part of the cube that still leaves you with a significant amount of freedom of movement without breaking what you have already done. There are several ways to do this. My favorite method (Roux) starts by not making a full layer, but just a 3x2 rectangle on one side. This rectangle is placed on the bottom left part of the cube. You still have a considerable degree of freedom, you can turn the top layer and the two rightmost layers without breaking your 3x2 rectangle.
The next step is to build a symmetrical 3x2 rectangle on the lower right side of the cube. This is quite easy to do by just using the top layer and the two rightmost layers, thus avoiding to mess up the left hand 3x2.
After finishing the two 3x2 rectangles (commonly known as the "first block" and the "second block"), the next step is to solve the corners on the top of the cube. This is the only algorithmic step of Roux, you use a number of memorized algorithms. However, the algorithms are shorter and simpler than those for the top layer of a layer-by-layer approach, because the algorithms are allowed to mess up everything along the middle slice (which hasn't been solved yet) and the edge pieces on the top of the cube.
After finishing the top corners, you are still free to move the middle slice and the top layer without messing up what you've already done. Fortunately, this is enough for solving (intuitively!) the remaining pieces. You can finish the solve by using only these non-destructive moves.
The Roux method, therefore, allows you to keep the maximum degree of freedom of movement (without destroying what's already been solved) all the way until the end. This is what allows it to have a very low move count, and what's makes it easy to learn. It also gives you a lot of creative opportunities compared to CFOP and other layer-by-layer methods. Because of the increased freedom, there are more ways of doing things, and bigger scope for clever shortcuts, especially when building the first and second blocks.
It helps also to develop some sort of notation for yourself. This way you can track and repeat your moves.
Solving by layers is kinda logical. So solving one side (first layer) is not hard. Then some experimentation with rotation sequences which temporarily break the solved layer/face and then re-assemble it will lead to discovery of moves to swap the edges into the second layer.
The hardest then is to solve the third layer. Again, the notation and observations help charting your way through.
A curious discovery may be about some repeated pattern of moves which may be totally shuffling the cube yet, if continuing it, eventually returns the position to the beginning state. It's kind of a "period".
Have fun.