This is the basic pattern:

The body will look different depending on your measurements and how loose you want the shirt to be.
You'll need these measurements:
1. The length across your shoulders from one wrist to the other, with your arms spread out like a "T."
2. The width of your chest at its biggest part (you can measure around yourself completely and divide by two if that's easier).
3. The length of your chest from the center of one underarm to the other (I forgot to label #3, but it's the only gray line without a label xd )
4. The width of your waist (measure completely around and divide by two)
5. The width of the shirt at the bottom (measure around wherever you want the shirt to stop and divide by two)
6. The width of the sleeves
7. The length of the entire shirt, from shoulder to wherever you want it to stop.
8. The width of the neck hole (make it large enough for your head, but not wider than your shoulders).
Using inches: if you want a baggy-bodied shirt, add two, three, or four inches (your preference) to measurements #2, 3, 4, and 5. Then, add 1 inch to all the measurements to allow for seams and hemming.
Using centimeters: if you want a baggy-bodied shirt, add five, seven, or nine centimeters (your preference) to measurements #2, 3, 4, and 5. Then, add two centimeters to all the measurements to allow for seams and hemming.
These are the final numbers to use for the measurements.
Now you're ready to start!
Step-by-step instructions:
Start with a piece of cloth that's as wide as measurement #1 and twice the length of measurement #7.
Fold it in half lengthwise and lay it on a flat board, with the fold at the top. Fold in half again, down the middle.
Mark measurement #6, starting from the top folded edge.
Mark the other measurements, starting from the middle fold and going out half the measurement (essentially marking the boundaries of half the shirt)
Draw the shape of the shirt if you wish, or simply cut out the outline.
Unfold the middle fold to reveal the symmetric shirt. Sew together the sides and sleeves, and hem the neck, bottom, and sleeve edges.
Finished product:


(I'm not very good at giving directions, so if someone wants to give rewording some of the confusing directions a shot, go ahead sweatdrop )