It couldn't be easier. Text RESIST to 50409 (details at Resistbot) and follow the prompts.
This is what I sent:
As you are quite aware someone has announced that Transgender persons are no longer welcome to serve our country. Then, while the media is having a firestorm over that sudden announcement and as the senate GOP are dramatically playing out their "take the healthcare away from the poor and give the money to the rich" scheme, it seems the Department of Justice decides to submit an amicus brief stating that all LGBT Americans simply aren't covered under the 1964 civil rights act because of who we are and who we love. I know that you and all of this nation's governors, senators and representatives want to be on the right side of history. For that reason I urge you all to put party politics aside and work towards a just, fair and equal society for all of us. I ask that you stand up and fight for all of our rights as equal citizens no matter our sex, religion, race, sexual orientation, skin color, beliefs, age, pre-existing condition, income, or social status. Please do this for us, for yourselves, for our children and for our country. We need you.
Thanks to Fearsome Beard for providing the majority of the text. He very eloquently put into words what I wanted to say.