When trying to freehand peen your scythe blade, one of the most important elements is getting the lighting right so you can see the hammer marks on the scythe. Being able to see where your blows are landing allows you to adjust in order to place them accurately; if you can’t see where you’re hitting, your chance of success is much lower.
First clean your blade, anvil and hammer using a fine abrasive such as wet&dry paper or a garryflex block. As well as removing any dirt this will leave a matt surface on the edge of your blade. Then each strike of the hammer will shine the surface of the steel, the photo shows these marks as I hammer the first row of peening onto a new blade.
To see those marks you need low angled light from in front of your peening station, I find evenings to be the best for this which also makes a good end to a day of mowing. Experiment with your own situation and don’t be afraid to move yourself around to get the best view.