Help Needed! Characters Walking Along The Grid Thread

So I'm trying to make a fan game for Markiplier and I am having the worst time with the coding! Well, not the "worst" time; I've got most of the basics down. Nevertheless, I can't figure out how I'm supposed to code the characters (Mark and the NPCs) to walk along the grid. I know how to make them move with the WASD and arrow keys (I'll be using WASD in the game), but either the characters' movements don't conform to the grid, or they do, but the movements are choppy and it snaps the character to the grid in such a way that he won't progress a square if you don't hold the key long enough. I am unable to progress any further in the game development as a result of this major setback. If you know how to fix this problem, I would be forever grateful to you and will credit you in the game.

On a side note, I am using the Game Maker Studio from Yoyo Games, if that affects how the code would look.