Can I substitute frozen spinach for fresh? Thank you!

Thank you so much. If I am using frozen spinach, how much would you suggest using?

You surely can. I have just 2 pieces of advice in that regard: 1. Thaw the frozen spinach thoroughly before using, and wring it out in a cloth tea towel VERY well. and 2. Buy frozen whole leaf spinach rather than chopped. You will need to chop it yourself after it's thawed, but oftentimes chopped spinach has an overabundance of stems in it... you'll get more actual spinach, and more attractive product if you buy the whole leaf spinach.

Absolutely, just wring the water out of it after it's thawed.