the first post by Soeren briefly details the steps...but all expand a little...
In VM, parent/child products allow stock levels based on attribute differences.
we agree that the attributes will be used to define the different versions of the child products.
VM may be a little more difficult to set up with respect to parent/child products than other carts, but allows MUCH more flexibility. You will see once you get used to it.

terminology:
Product = Parent Product
Item = Child Product
note: you will never sell the parent product...only child products if they exist. So, you may want to define the Parent SKU in a generic fashion (in your case, maybe parent SKU would be "CM000") and the parent price might be set as the price of the lowest priced child, so the customer sees low price and gets excited... you make the decision.
How to set up child products (items): ===========================================
I will detail your first child product (trail size Cleaner Milk) the rest is up to you...just repeat!1. create a product (or use an existing one.) this will be the parent product.
(so, in a sense, all products are parent products.)2. add an attribute to the parent (eg. Size) by clicking the
Add Attribute button at the top of the product form. Type in "Size" in the
Attribute Name box..( you can also set the list order if you want)...then save.
3. Go back into the parent product form. Click the
Add Item button at the top of the product form. This brings up a new product form to enter the details of the child product.
4. Put in your unique item Name (Cleansing Milk - Trial Size), SKU (CM30t), Item price (5.95) (yes, also quantity based prices as well if you wish), and descriptions on the first tab of the form.
(Note: Notice how the tabs on this form are named "Item Information", Item Status", etc..instead or "Product Information", "Product Status"...? this indicates that you are editing a child product (item.)
5. Then click over to the second tab, "Item Status". Look toward the bottom and you will see a text box to fill in labeled, "Size". This is the attribute you defined in the parent product. Now type in the "Size" of your Item. (in this case, 30mL)
6. Continue to fill in the remaing data on the rest of the tabs (if you want.)
7. Click the Save button at the top. You will be shown a summary of the Item details.
8. Click "Product List" link on the summary and you should see the entire product list. Look at your Parent Product, it should have "[Item Information]" link next to the name.
9. To add another child (item) go into the parent and repeat steps 3 thru 7 above. OR, to view the list of that Parent's childs, click the "[Item Information]" link next to the name of the parent.
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The way this cart is designed gives you so much flex....you can make a child item have its's own description, look & feel, weight, yada yada...AND keep track of inventory...unlike other cart software!