I am going to tell you how I use simple means to sharpen a knife with Scandinavian sharpening. It does not matter how sluggish it is, if I do it right it will always be sharp.
What I need is: a metal file, sandpaper with different sizes and a flat surface and a little finger feel.
(If you rather watch a video, link is below. It´s in swedish.)
When to sharpen a knife, I start to check if there are any large notches that need to be filed away, if so, I use a metal file. If the edges of the ridge of the blade are round, I sharpen them so that I can strike sparks on a fire steel with them. I do not want to have to use the edge of both the knife to strike sparks because then the knife becomes dull.
When larger notches are filed away and the edges of the ridge are sharp, I start with the edge.
I start by attaching my coarsest sandpaper to a board that I attach to a flat surface with double-sided adhesive tape. Then I press down the edge side with a couple of fingers so that it lies flat against the sandpaper.
Then pull the knife with the blade back forward so that the edge does not cut into the paper. I repeat this several times until I think the edge is free of notches and notches then I turn the knife and take the other side. The roundel up to the tip, I am a little easier on the hand with because it curves. When the sharpness looks good, I change to a finer paper and repeat the procedure. Last time I sharpened my knife I used paper with the coarse sizes, 240, 400, 1000 and 1200.
The choice of paper depends on how damaged your knife is and how shiny you want it, the shinier you want it the finer the paper. If you want it to be really shiny, you can use a polishing cloth or some polish. Good luck!