Normally, a scraper is sharpened by "draw filing". It's done with an 8" mill bastard. This means the file is held perpendicular to the scraper, not straight in line, only 2 or 3 strokes are needed. If you take more strokes, there is the possibility of not keeping the edge straight. You only file straight in line if the scraper gets out of straight.
Using stones is a waste of time with a card scraper and often counter productive. The burnisher does all the necessary work but you need a real burnisher that is hardened and polished smooth. Pieces of hard metal that some like to use will not do the job correctly.
The key to scraper sharpening is drawing the burr, then turning it. Drawing is done by laying the scraper flat on a surface, then working the burnisher on the flat sides of the scraper, not on the edge. This mushes the metal ever so slightly and gives you the compressed, sharp edge that will then be turned into the cutting burr. Files and stones leave striations on the edge of the scraper, the burnisher smooths these out when drawing, similar to the way a chef uses a steel on knives. A butcher's steel doesn't remove metal, it burnishes it. You can reform the draw and turn a new edge several times before needing to re-sharpen.