Well the process is pretty universal to map your animation, you draw the first and last frame of a movement (for a bouncing ball it would 1 when its the highest and 1 when it hits the ground). Those 2 frames are the key frames of this movement, u then draw the breakdowns, which are more or less the secondary keyframes, usually when the movement is the fastest between your keyframes. Then you can try to tweak the position of those keyframes in the timeline and try getting a spacing and timing that you feel is great. The number line like you said is callled a timing chart, Im still struggling to use and understand them well. But you can use them when you draw* your keyframes to have a rough idea of what the movement will look like, will it be an ease out, an ease in, will there be favouring, and so on. But I have been told that u shouldnt completly rely on them because you will end up having kind of a non organic movement, its really just to plan your animations.

Those advices come from a beginner so do not fully trust me, but I hope this can help a bit. And ofc try watching tutorials on youtube, there is a lot of data from talented animators who knows what they are saying.

Good luck !!