Easy, I built a portal between Earth and Equestria, I then got to know the ponies of Ponyville including the mane 6. Curious of human culture, Twilight asked if she could see Earth and I said sure. Last night she fell asleep in my bed and I took a picture.
But in all seriousness, I threw some pillows and blankets on my bed in a way that when covered with a blanket, would look like a pony. I made sure the blanket curved in a way that would make it look like there is a neck coming through.
Then I took a picture wtih my phone (Droid Bionic 8MP camera) and put the image on my computer.
I opened the image in Easy Paint Tool SAI and got to work drawing Twilight, taking care to make sure her shadow matched the lighting in the image.
Last but ABSOLUTELY not least, I saved the image as a .JPG rather than a .PNG like I normally do. That way, Twilight gets a bit of grainyness to her like the original photograph, this is to lower the quality of the drawn portion to match the photograph better, thus making a more convincing image.
As for why she seems long, one of the pillows I used was a body pillow and it's probably 150% the lenght of Twilight Sparkle. So she seems long.
But I'll just chalk it up to her having a really bushy tail.