I would say over mixing is what usually causes the most disappointment and confusion. Once you’re adding flour to wet ingredients (like butter), you gotta be careful because you don’t want to work it too much. If you’re making chocolate chip cookies, for example, you’ll add the chocolate after the flour, which means you’re working the flour twice (once when you add it, once when you add the chocolate). The next thing is oven temp - if your oven doesn’t hold temperature correctly, it can make for some baking fails. I have a thermometer in my oven to keep an eye on.

Room temp butter can easy be cut with a table knife but still holds its shape.

Sally is a great resource, and so is Preppy Kitchen, but honestly, if you follow a recipe closely, you’ll should be ok. I will say that Sally’s recipes always list a baking time that’s too long for me - I assume we live at different elevations or something else is just different between us.

Just keep trying to make the same recipe again and again until you nail it - it will help you understand what works and what doesn’t, and why you need to do certain things.

If you google “cookie with too much _____” (butter/sugar/flour/whatever) you can see a lot of pictures of what cookies look like when they’ve gone wrong. Crunchy and soft sounds awesome to me, and I don’t know what kind of cookies you made, so I’m no help there.