How would I susbtitute fresh spinach for frozen?

Frozen spinach has been boiled/blanched. You can do this if you really want it to be as much like frozen spinach as possible, but really, you can just cook it with the water left on the leaves from washing - effectively a bit more like steaming. There's no need to freeze it. This will result in something with fresher flavor and a bit more substantial texture than frozen spinach (not as eager to disintegrate). And you don't have to worry about moisture much - a lot of the water will cook off - but you can certainly drain it additionally if it's too much for you.

If you really want it exactly like frozen spinach, you could boil it, and maybe even freeze it to help mess up the texture, but I'm guessing you'd prefer cooked fresh spinach anyway. In this case, you'd have to squeeze and drain it just like with frozen spinach to get the water out. (If it's unclear how to do this, see What is the most efficient way to squeeze water out of cooked spinach?.)

Based on nutrition facts for raw spinach and frozen spinach, one 10 ounce package is approximately the amount you'll get from cooking a 340g (12 ounce) bunch of spinach - that weight is probably after removing the stems you're not actually going to cook, though. This fits with my experience cooking down spinach. Most things you'll use it in are really forgiving, so probably best to err on the larger side with your bunch of spinach!

I don't bother with the steaming or any preparations when substituting fresh for frozen. If the recipe calls for 10 oz. of frozen, thawed and squeezed, I simply de-stem and chop fine my fresh pre-washed organic spinach till I have about 12 oz. in a glass measure (1 1/2 c.)-way better than steamed, frozen thawed, and better tasting.

I just made a spinach lasagna recipe that called for frozen spinach. I used about 9 oz. (bag) and chopped it up without removing stems or anything and just mixed it in with the cheeses. It worked perfectly.

Based on the calories in one 10 oz package (3.5 servings @ 30 calories per serving = 105 calories) compared to the calories in 1 lb fresh (4.5 servings @ 25 calories per serving = 112.5 calories), one 10 oz frozen is almost exactly equal to 1 lb fresh (trimmed and mushy leaves removed).

Trimming all that raw spinach is time-consuming, but squeezing thawed spinach isn't my favorite activity, either. Plus a good fraction of the frozen-thawed spinach goes down the drain because it sticks to my hands or the colander as I squeeze it.

With fresh spinach: I remove tough stems, blanch it in boiling water for 30 seconds, drain, rinse with cold water to cool, allow to cool some more to get to room temperature, squeeze into a ball - it's much easier to work with than pre-chopped spinach - and then slice the ball and slice the slices to end up with chopped. That's the procedure used in James Peterson's Vegetables cookbook to prepare the spinach for an Indian Vegetable Stew and in my opinion the result has better flavor than thawed frozen.