Recipes are just guidelines. With enough trial and certainly a bit of error, it will become more and more instinctual to know which substitutions will be successful and which ones will not. A great way to learn/practice is to try and use up leftover items that may be hanging around in your fridge, yet still in their prime. Extra herbs, leftover pasta, rice or some cooked squash are things inevitably found in my fridge just waiting for another chance. As a result, all kinds of pestos, veggie rice bowls, mac n'ham n'cheese and squash quesadillas have emerged from my kitchen sans recipe. (Let it be known that I am speaking B.C., as in Before-Cheffing, so at the time there was no such unfair advantage.) Recipes are useful to familiarize the cook with techniques and steps but using them solely as a reference, once you are comfortable enough, will give you alot of freedom to experiment with your own ideas.
Another important lesson that will go hand in hand with recipe off-roading is learning how to fix a dish in case you have gone too far and things aren't looking so delicious. Tuning into enough cooking shows can help prepare you for such culinary tests, though I recommend 'actually' cooking as opposed to 'watching' cooking. That said, it all helps either way. I credit food television with exposing people far and wide to exotic dishes and ingredients, which is also a great start. Try to be inspired by a dish you have seen elsewhere and apply it to something you make for yourself. As Josh said in the previous post, you will learn from your mistakes and become a better cook in the process.
But what good is all of this learning without a little homework to reinforce the curriculum? So, I leave you with an assignment in the form of a vague recipe that I want you to try. Make it taste good and you have passed...
1 can of beans
1 splash of an acid (citrus or vinegar)
a handful of nuts
a pinch of a spice or an herb
a glug of oil
Spin the ingredients you have chosen in a food processor until smooth. Taste. Adjust. Taste again. Adjust until satisfied. Serve as you would any other hummus that you have purchased at the market... but would not feel as rewarded eating.
One last thing: Let us know if you try this out and post your results, will ya?