In the simplest case, the easist way to sharpen a molding plane blade is to just flatten the back. This is a method that will preserve the profile and still bring you to a sharp edge. Generally that is the safest course and pretty much guaranteed to work. Molding planes generally get a lot less workout than a jack plane or a smoothing plane. Since the profile is complex, you do not look at the surface in the same way you would from a smoothing plane.
If the blade is nicked or does not match the profile or has been ground to vary from the profile, then you need to reshape the blade. When the blade is fixed in the body, you should be able to see an even projection of the blade above the sole. Wherever it is not even, you will either have a too heavy cut or the plane will stop cutting when it hits that part of the profile.
Paint the non-beveled side of the blade with machinist's bluing ( I use the red version), install it in the plane with a good projection. then take a pointed steel scribe (like an awl) and laying this flat on the toe portion of the sole, carefully scribe the profile onto the blade. Don't tip the scribe up to do this. Then with your grinder tool rest set to 90 degrees, grind the profile square to the tracing. I have some 1/8" and 1/4" wide grinding stones (Norton pink 60 grit) for inside curves. For outside curves you can just use a wide grinding stone. Once the profile is accurate (check it in the body several times), go back and set the tool rest to 25-30 degrees, and make the bevel, creeping up on the edge and cooling the blade frequenyly. After that, hone the back then the bevel using a 1000 grit slipstone.
An alternative approach is to use a Dremel tool. They sell carbide bits which will cut the steel. Also a wide variety of grinding bits. That requries a lot more hand coordination, but will get the job done. Depending on how hard the steel blade is you might be able to use a chain saw file ond varioius flat files.
You can see why once the profile is established, you do not mess with it again, just focusing on the back!