Apple - How do you position your hand to use the command key? What finger do you use?

You can use either thumb to hold down a ⌘ key and then use another finger to access the other key. You just have to get used to sliding either thumb a little to the left or to the right of the space bar.

Yes, this is just a result of years of using a PC.

Here is the official advice from Apple

On page 67, they state the following:

Use a light touch when typing or using the trackpad and keep your hands and ngers relaxed. Avoid rolling your thumbs under your palms.

Based on this, it would be my assumption that they intend you to either move your entire hand and use your thumb or pointer finger. If you find this unnatural, you can always change the control key to perform the duty of the command key. Open System Preferences > Keyboard > Modifier Keys, and change the buttons around as you see fit.

When I first made the switch to Mac I used System Preferences -> Keyboard to switch the ⌘ key with ^ (control) key. That worked for a while until I became more comfortable with the basic two-key commands. Once I started using more complex three and four key commands I had to put it back to default because I needed the labels on the keys at first to remember what the symbols <-> key mappings were.