Here's something that might be useful. It's 30 years old, from the January 1978 "Red Book" Apple II manual, but it addresses exactly what you're looking for.
http://apple2history.org/dl/redbook.pdf
See pages 114 through 120. Since you want TTL, you don't need the RS-232 converter circuit shown. There are a few OCR errors in the code listing (most obviously "3" where it should be "B"), but it ought to be helpful. There's a numeric value on one line that is chosen to produce a 9.9 millisecond delay for 110 bits per second. Changing that constant should change the baud rate.
Have fun!