I would think that just concentrating on and saying with love the vocal prayers we already say would constitute mental prayer, or at least would be a good start.
That's what St. Teresa de Avila did with the Our Father.
I pray the rosary with the second method of St. Louis de Montfort, and that helps me focus on the mystery I'm reciting and pray for the grace to imitate Christ and His Mother. I also dedicate the rosary before beginning it and sometimes I reread the scriptural passages associated with the mysteries before or during my recitiation.
I dialogue with God when I'm recalling my sins before praying the Act of Contrition each night, and I make specific requests to Mary when I finish the Memorare. I have images of the Sacred Heart and a crucifix over my bed and I meditate on them when I say my morning and evening prayers and especially the Prayer Before a Crucifix. Those are very simple things, nothing fancy or profound, but I believe they do constitute mental prayer, and I'm pretty sure we are all doing those things or similar ones already.