Warning! The following contains minor campaign spoilers! No story spoilers though!
I made a tutorial on how to launch a warthog on the mission Glassed. You can either watch the video I made(feedback appreciated) or follow these instructions:
"First you load up the third mission of the campaign called glassed and then you get to the point where go outside of the Meridian base and have to clear out a lot of Prometheans.
Once you have done that there will be a warthog waiting for you. Now all you need to do is set the warthog into position on the gate(no wheels on the gate) and get out of the warthog. Finally you need to step outside to trigger the gate and then you simply get back in your warthog. Select ‘Resume from last checkpoint’ if you want to give it another try.