Grain size is changed in heat treat, and it can be increased or decreased quickly and fairly easily (a little too easy to blow it up) The only thing I can think PR is trying to say is that carbide size is affected in the melt, that is more correct. It would be entirely pointless to produce 52100 with PM, the carbide size and distribution is already finer than what PM produces. 52100 is also hardly wear resistant when discussed in the company of PM steels produced for plastics molding, which is mostly what we see. It's a fine steel, but 1% chromium isn't burning up the charts with carbide content.
You would go to other forums for more information. This is a somewhat active forum, but there is a wealth of info outside of BF. Woodworkers, straight razor shavers, foodies and chefs, professional sharpeners, stone retailers, stone miners, synthetic stone makers, sharpening system developers, etc. - they post in a lot of places other than here. But, I wouldn't worry about that info quite yet. Your main concern right now, if you want to freehand, is to just work on technique. Technique is easy, hold a consistent angle and don't go too fast.
I personally would start with diamonds. It eliminates worrying about having a strong enough abrasive, keeping the stone flat, loading up, mud usage, soaking, oil vs water, chips and cracks, etc. A splash of water and away you go. Rinse it off if there's a ton of swarf.
Knifenut is right about diamonds not being optimum for all steels, but he's working at a level well beyond where you are. If you just want a sharp edge for regular use, a coarse and fine pair of diamond plates ought to do it. You can progress beyond them later. A little cheaper would be some Norton Crystalon or India stones.