Adding lemon to cookie recipe
The easiest way is just use a recipe for lemon cookies. I'm sure you can find lots of them online. If you really want to use a recipe for normal cookies (because you really like it), you could certainly add lemon extract to a regular batch. Don't add too much, the flavour is really concentrated. Since you won't use a lot of it (a teaspoon will do), the liquid:dry proportion practically won't change, but to be sure, you can cut back the milk by the amount (e.g. a teaspoon). You can also add a bit of zest without a problem.

I wouldn't add lemon juice because you're adding milk in your cookies. The milk would curdle (in some cookies that is exactly what you would want to achieve). However, you can try to add lemon juice instead of milk.