Nancy’s answer is spot on. You could try to teach a strength training class by reading a book about it, or taking a 3 hour workshop, but if you have not done it yourself it is so much harder.
The first thing you want to consider is what type of mindfulness technique you want to teach. Meditation is the most common, though not the only approach. There are a bunch of styles: Kabat-zin’s mindfulness meditation, shambala, Zen,…. oh lots. There are probably groups in your area you could join. If you like online one person I have had a workshop with and liked is David Nichtern. He is a senior teacher in the Shambala lineage, and he does free dharma talks online once a week, or used to.
Unless you want to teach in a specific program or lineage you aren’t going to need a specific ‘certification’, but be aware an awful lot of people are teaching it now, including medical people, therapists, as well as meditation masters, and you will likely be asked by prospective students about your background and your own practices. If your competition are a Zen priest with 20 years of meditation training, and a Psychologist with a PhD and training in cognitive therapy you want to have some substantive training as well. Certainly being a yoga teacher is one route, as mindfulness is and has been since the Yoga Sutras were first written, a central part to this practice.
You could decide, rather than becoming a master of one technique to learn about a range of them and teach beginners about different ways of being mindful. I have a beginner mindfulness workshop I do from time to time, a kind of sampler to let people see the range of applications of mindfulness, and then to decide what type of practice they most want to follow. There are exercises for mindful eating, for journaling, for sensory focusing, as well as for meditation. This is something I developed through years of my own practices and study, as well as through a lot of work with different teachers.
I would also like to recommend the book “Pen and Bell”. One of the tools in the mindfulness tool box is journaling, and that book has wonderful practices.