When To Harvest Herbs

This is the first time I have tried growing herbs. My question is, where do you take the leaves or stems from the herbs. For instance basil, do I snip the slower leaves or higher ones. Or lettuce, I'm growing romaine in a pot. I'm snipping off the largest leaves, is this correct. And when you do snip parts off do they regrow or do I have to plant more?

You harvest herbs according to their growth habit. Basil as well as rosemary, thyme and sage, you cut off the tips closest to a lower section with new shoots showing. You did not mention the quantity you will need each time, but cutting off 1/3 is a good rule. Of course if you need a few leaves cut off the upper section, the next time cut off to the next section down to a third of the height. The oldest, lowest leaves tend to turn brown, so discard them
Romaine lettuce grows the new leaves from the inside, so you are doing the right thing by taking the oldest leaves first. The same goes for parsley and cilantro.
If you need a steady supply, it is a good idea to grow new plants at intervals since both the basil and the lettuce are short lived herbs.