Sharpening is a semi-toxic issue in these parts..... but with a shoulder-plane, apart from it being sharp, one thing is important - with the blade in its cutting position and sitting square to the mouth, its essential that there's a slight overhang of the sides. Only about 0.5 mm, if that, so that the corner and the edge remain true in the corner of the shoulder. Therefore the width of the cutter will be important.
With that plane, it was common to match the blade width to the actual plane body - you will probably notice that they are both numbered at the factory. The batch number on the plane body would be matched to a blade and then it was numbered.
The sharpening needs to be keen because this plane is intended to cut cross-grain on tenon shoulders, as well as possibly cleaning small rebates.
So, in three stages,
1 - Flatten the back (flat) side then polish down to a shiny finish. You only do this once then keep it maintained. Essentially it needs to be clean, flat and the strip alongside the back of the bevel needs to be dead smooth - this is one part of your cutting edge and needs to be good.
2 - Sharpen the beveled side square to the sides though all your grits to a good edge with the thinnest hair on the cutting edge that you can get - strop this down on a strop if you have one and remove the whisker. You may, or may not, favour a jig at this point ( I don't use one) but it's worth pointing out that mounting this shaped blade in a jig is a monumental pain and jigs were probably not invented when this plane was made.
3 - Dress the sides of the blade. Lightly stroke each side of the blade on a very fine stone - don't take any metal off! - until it is smooth and you have a sharp point at the corner where it meets the bevel. You only do this once, then maintain it.
At this point, the blade is good to go - a little wire wool to remove any krud on the inside seat of the body will ensure a snug fit.
That's my method...... I hope that it works for you. As I said before, sharpening is a semi-toxic issue hereabouts, so there will be variations.......