You can't free-hand or guesstimate baking a cake. Regardless if you're a first-time baker or Buddy Valastro, the Cake Boss himself, you simply gotta stick to the recipe.
Cakes are more than a random mixture of flour, sugar and eggs. They're a precise combination of ingredients, and if you don't have the recipe right there in front of you, you're more likely to craft a puddle of goo than a firm, attractive cake.
That said, don't be afraid to improvise. You can make some substitutions, and if you're an old hat at baking cakes, you can always throw a little something extra into the mix. For instance, you could try adding lemon zest to a pound cake recipe that you mastered years ago. No matter what creative additions you make to the batter, just be sure to use the recipe as a guide.