"Do I use the whole plant stems and all since I will be finely grinding it?"
When you are making coconut oil, you will want to use cannabis flower, and make sure it is ground up nice and fine. Most people will dispose of their stems, but there is nothing wrong with adding them into the recipe, especially if it is broken down into very small pieces. You will need to sieve the marijuana from your coconut oil mixture, so by the time you have your finished product, all stems and leftover flowers should have been removed.
Here is a great article with instructions on how to make cannabis-infused coconut oil. You can adjust the measurements to your needs.
I hope this information helps! 🙂