If you don't have applicable product options already set for products you will first need to define product options for products in your shop system (Shop --> Product Data --> Product options --> Option Add).

Once the applicable options have been set up in the shop system, you can indicate which of those options apply to the specific product on that product's Product Options tab.

To generate child skus for the different options go to the products SKUs tab, choose an option and click the purple 'create SKU' button. A Sku will be generated for that product option.

Each sku can have it's own pricing and image. To update the sku pricing and ensure that it can be purchased, click on the products SKU Pricing tab. The box under Can Purchase must be checked for that option to be purchasable and enter the price for that sku.

If you are tracking inventory, the sku inventory count can be entered on the SKU Inventory tab.
If you have different images for each sku (i.e. color) those can be entered on the SKU images tab. If you upload SKU images, then the image will change to that image when the shopper chooses that option. (i.e. be shown a blue shirt when the blue option is chosen).

The SKU Document Downloads is intended for pdf files specific to that SKU, such as an owner manual, special instructions, etc.

If you have a lot of skus, the quickest way to check that everything is set for purchase, pricing, etc. is to download the Price & Inventory file (Shop --> Price --> Price & Inventory)