How do I make my Loomie have my freckles/wrinkles/etc?

You can slide the Texture Control to the right to include more of the texture (wrinkles, freckles, etc.) from your face. Note this will also bring in any shadows or reflections from your image, so it's best to use a selfie with even lighting.

How Do I Use Loomie Stickers in GBoard? (Android)

You'll need to install the GBoard app and, in your messaging app, set it as the input keyboard(consult your messaging app for details). With GBoard set up, this movie shows what it looks like to edit and share a Loomie out as a sticker-pack and paste into a message: