Start with doing your research. Don't rely on stereotypes or misconceptions. Not all gay men are interested in fashion. Lesbians do not hate men. Bisexuals are not incapable of having monogamous relationships. Don't perpetuate myths and remember that everyone is different. LGBT people also experience different environments depending on the area in which they live, their family, and their friends. Some are only out to certain people, some are out to everyone, and some aren't out at all.
Your characters don't have to have a label. Sexuality can be fluid and not everyone wants to put a label on it. Some people might have preferences outside of their orientation, like a pansexual with a preference for women or a bisexual with a preference for men. A bisexual woman dating another woman isn't a lesbian; neither is she straight if she's in a relationship with a man. That doesn't change their orientation.
Sexuality is merely a part of a character. Don't let it be their defining feature. Make it feel natural, like heterosexuality does to most characters. Don't shoe-horn queer characters in just so they're included. Write LGBT characters because that's who they are.