Please follow below steps and GIFs. 


Steps:  

1) Select your Desired store you want to create an item in.

2) Go to Store Items >> New Store Item

3) Enter require information (Name, Unit, Item Category)

4) Activate item in Desired stores by select store and clicking on Active Button.

5) Go to Unit Tab >> Click on "New"

6) Select your desired UNIT from list. 

7) Put Conversion Factor. (Example:  1 Litter = 1000 ML) If you are NOT sure about conversion factor please follow this link (www.unitconverters.net)

8) Save New Unit tab.

5) Save Add new item.  That's all.