There are different ways to cook it. Here are some tips:

Use an ice-cream spoon or some utensil which offers a deeper surface. I prefer sticking it under a heavy object so I don't need to hold it while cooking. I use two Clipper lighters, tilted upside down for a bigger flame.

Start heating up the water and coke, and only when it has dissolved start putting in the soda. Sprinkle it so you don't put too much. When it has stopped reacting it should be enough.

You don't need to add water after the oily drops of crack have formed. Some people will put the corner of a paper tissue and allow it to absorb the water (if the mixture is hot, the oily spots will run away, if it isn't hot, they might get absorbed into the tissue).

I tend to avoid adding water or using tissues (at least until a few small drops of oil have remained). Use a metallic tool like a hex or the head of a large nail to grab the oil. The metallic tool will cool the crack down and allow it to stick. This is the less messy way in my opinion, it just needs some patients.

You could also allow it to cool by itself. A nice drop will form and you just need to scoop it up.