Plant them in the sun, water well and dead head when needed. There is nothing else you can do.
But you are about 2 months late in planting these. Usually your annuals go into the ground as soon as the last frost date has passed. You have missed 2 months of the growing season, they won't make as nice a show as if you had planted them about May 15th.
Linda C

Pentas were used as houseplants before they ever got popular for the garden. So, since you are planning on keeping them in containers anyway, just bring the pot/s into the house for the winter and see to it they get a sunny window. They're a perennial, but not in your zone. They likely would not have done much for you right after frost-free date anyway. They love heat and early spring in Massachusetts isn't exactly hot.

Thanks, Calliope. Yes, early spring in MA is chilly--rarely, it even snows in May. So tropical-loving plants don't do well planted that early. Anyway, the point is that I just found these yesterday because I was looking for something for a new container I bought and had a hole to fill. I wanted something different and these caught my eye. Alas, the container is way too big to come into the house. This is an experiment this season and I'll buy them again if they do well. Just wanted to know about growing preferences.

They are a wonderful plant. We used them last year for the first time in our county fair display. We started from plugs in April, eventually potted up three to a one gallon pot to make a complete flower cover for each pot. Fair rules insist on no edges of pots to show. They held up for two weeks in July inside a steel building with no air conditioning and still looked good. The ones we did not sell at the end of the fair sale I took home. I took each plant separately out of the pots and planted in my garden. They all recovered nicely and continued to bloom until frost. I took a few of the best looking and repotted and kept in the greenhouse through most of the winter. I did eventually discard them as I needed the greenhouse space. We have a new crop ready to put in the fair Friday and I will be sure to bring more home this year at fairs end, even if I compete with our customers and BUY them. Al

That sounds wonderful, Calistoga, but I see you are in CA and I am in MA so we may not have the same results. I used to have beautiful Madagascar periwinkles when I lived in CA but not in my garden here. Summer is too short and often not hot enough.