I speak french, forgive my English 
I created an item in my inventory. I do not buy my products, but I make them myself. So how to enter in my inventory?
Go to customise and add manufacturing orders. Use that for buying your parts that you need to build your products.
If I make jewelery, I have to count beads by bead or ???
Have a look at these videos. they cover inventory kits and manufacturing orders. This will help explain what would work best for you.
I make jewelry. So I understand I register my necklace in the inventory and then I create a manufacturing order for my necklace, each bead and each small pieces: O