@neriah Sometimes I also know a letter or word is not formed correctly, but am unsure of just why without careful study, not having the keenness of eye of a master of the form like Ken Fraser or a teacher looking over my shoulder to correct me as I practise, so here is the method I use to more quickly approximate a well formed letter.
1. Select from your exemplar that which you wish to practise and affix it to the appropriately sized guide sheet.
2. Lay a blank piece of paper over your guidesheet with exemplar.
3. Noting where in your exemplar where the stokes begin, end, and intersect the guidelines, make your first attempt next to it (as example, I'm making an especially poor first attempt).
4. Move your top sheet such that the letter you just drew is directly over the exemplar and it becomes obvious what is wrong with your letter (in this example, a lot of things are wrong).
5. At this point, I pick one thing that is off, and in my second attempt, try to improve just that element. Trying to improve everything at once when there are many things off seems to lead me down a winding path that something is always off (in this example, I work on the initial downstroke--shape and width are really off).
6. Compare your second attempt to the exemplar by moving it over the exemplar as you did the first attempt.
7. If it is a good reproduction of the element you're working on, move on to the next element that is off (say, in this example, the left loop--too narrow, wrong slant). Continue working through each element that differs from the exemplar until you have corrected them all (the last picture is not the final attempt, but I can only post 6 photos at a time. The process continues until all the faults in the last picture are corrected).
You don't have to move every attempt over the exemplar if you know what your working on is not right and know how to change it. When you think you've got it, then check your attempt versus the exemplar.
I sometimes begin this exercise by tracing the exemplar to begin the muscle memory process in forming the letter correctly.
This whole process may sound tedious, but it actually goes quite quickly, and is certainly faster (for me) than endlessly forming the letter incorrectly until I eventually correct all the errors at one go.
This process can also be used for entire words and sentences, which allows for corrections in spacing, uniformity of shading, and other aspects not germain to writing a single letter.