I’ve done this twice. With the proper equipment, the task is not difficult but it is time consuming. There are several YouTube videos on the topic.
Corded and battery-powered drills with heavy grit sanding pads and wire wheel brushes. Also, a detail sander for the corners and sides. A Dremel is nice for getting the grease drain opening.
Grind off all the build up, blow off the debris as you go to see what you’ve completed, check for quality of work and to give your hands a rest. Once you’re done and happy with the removal work, dry wipe it down until your rags / paper towels come up clean.
Give her the gas, Chester, and heat up the griddle surface to max heat. Apply water until you’re reasonably sure you have removed all the crud. Leave the heat on until the surface is dry (won’t take long).
Apply your seasoning oil until you’re happy, run out of beer or booze or all of the above.
I have found that flax seed oil works the best for seasoning the griddle surface.
Good luck.