The best? A low speed water wheel with a proper angle guide is my choice for scissors
But short of that? I'd recommend a fine diamond plate (will stay flatter than any stones), a steady hand, and touching up with just a few light strokes often.
Easier to maintain an edge than set a new one (cheaper too, as you won't need multiple grits that way)
Keep the bevel angle exactly as is from the factory.
Sharpen the bevel only, never touch the flat inside faces of the blade.
Gentle and even strokes for equal grinding along the entire length of the blade.
Before use, remove the burr:
Depending on the construction of the scissors, if you can get them closed without the blades touching (by applying pressure with your opposite hand to keep the blades apart until they're closed), do that then open them, repeat 2 or 3 times.
If they're too tight to allow that, then remove burr by dragging each blade through a cork, or into the end grain of a soft wood, to remove the burr before re-assembly