It was full of visual advice—to see the line work, the brush work, the ben day pattern shading (where you see the yellow-brown splotches is the remaining adhesive from where the ben-day dots once were, but over the years have fallen off).
The white-out! What a lesson. Lots of white-out! Not every line has to be perfect the first time around!
And the size. I knew already that cartoonists worked larger than print size, but 300%? This art was 3 times the size of what we saw in the paper! It's amazing how the detail holds up in reproduction. Below you will see the comparative sizes.
I see by the date on the letter that I was only 15, but yes, I had submitted a comic strip feature idea to a syndicate. It was to be stories about science—not fiction, but fascinating portrayals of true stories of science. I sent a synopsis and some samples of the first intended story, that of Percival Lowell and his amazing perceptions of the canals of Mars.
I really think I did a decent story line and my art samples don't even embarrass me now. Yet all I got from the syndicate was a form rejection, supplying no encouragement whatsoever.
Hearing from Milton Caniff, though, was hugely encouraging—not knowing that I was fulfilling a cartoonists' tradition, as experienced and portrayed here by Caniff, himself: