If no artist is listed, no letters are noted. You won't put a +1/+1 counter on Abstract Iguanart.

If the artist is credited in a language other than English, use the first character that appears (even if in that language, the character isn't a letter, strictly speaking).

If the card has multiple artists listed (usually separated by &), note the first letter of each of them. For example, if you cast a spell illustrated by Zoltan Boros & Gabor Sziksai, note the letters Z and G. If neither letter had previously been noted, you'd put two +1/+1 counters on Abstract Iguanart.

The first letter of the artist's name is the first letter of how that artist is credited. For example, the D in Deruchenko Alexander, the B in Brom, or the C in Claymore J. Flapdoodle.