I have done it several ways:
1. Print directly on the inside before the card is made/folded. This requires a wee bit of measuring to get it centered.
2. Use a stamp & stamp directly on the card or stamp. Sometimes I line the card with another piece of cardstock & would stamp on that. I usually do this if the card base is a dark color.
3. Use rub-ons directly on the card.
4. Print on vellum & attach the vellum with sticky dots adhesive sheets (from 3M, I believe). Basically I do similar to as I would with option 1, then fold the vellum in half, put adhesive just on the edge of the fold & adhere.
As for brads, I usually only use those on the front of the card & will usually put it on a top piece of paper & then the brads don't show on the inside of the card.