well, I keep using the onion skin from graphicgale so I can keep a track of what exactly changed, then after that I just go on and on, test on normal speed, if I see something wrong I try to see what frame is the one with the problem and edit it, sometimes I just add a small number on the top right part of the frame so I can keep see the frame with the error easier, for animations and such, if they are tons of walking animations, lets say 10 human male same size animations I just make one and reuse it to save time and work on more important aspect, it really depends on what you are making in top down games they artist decided to add tons of frames to the walk animation of the hero since it is something you always see, while on the NPCs they just uses three frames in which two are the same frame just flipped hah, if you are walking on plataformers well, you have to make all the animations look decent heh..
But in general I recommend you doing one base walking adn from that use it as reference on all the walking animations, same might work for the others like jumping attacking etc, either that or onion skin. Animation is a really long a tedious job, I am no expert at all so some of my advices might be noob-ish, but in all art related practice is a key, so keep trying and trying.