Your pediatrician's office can probably teach you - ours did, because we didn't want to make the trip to the children's hospital. It's easy, but you do need someone to teach you.
There are pre-filled pens of MTX available but I believe they are only approved for juvenile idiopathic arthritis and rheumatoid arthritis so far. You could try getting them off-label, but it may not work because they're most likely more expensive.
My daughters took folic acid every day - in the morning. After my younger one had side effects with MTX, her dose was increased from 1 mg a day to 2 mg a day.