Make sure the dildo you buy is harness compatible. The flanged base must be big enough to keep it in place. If it’s too small it will slip through the round opening. Firm dildos are easier to manage. Soft dildos feel good, but are often not firm enough for penetration. They seem to have a life of their own.
Explore the vagina or anus with your fingers before you penetrate someone. Then it’s easier not to poke the perineum, which is painful for your lover, and to know in which angle you have to enter the vagina or anus. But it’s also possible that the one being penetrated guides the dildo into the anus or vagina.
Use lots of water-based lubricant.
Ask your lover what feels good and what doesn’t.
To use a strap-on dildo well might take some time. To become skilled you need to pay close attention to the needs and limitations of the receptive partner. That’s your only feedback.
Never use a dildo first in the anus and then in the vagina, for this will cause a vaginal infection. To prevent STD’s you should not penetrate different persons with the same dildo, unless you cover it with a condom.