This is
A variation of the Zodiac Z-340 cipher. This is a "S-340" cipher because the author's username is Sny Smartie.
To solve the cipher, you need to
Use the same trick as the Z-340 cipher, going through the grid with L-shape movement. There is a twist, though: every 4 movements (roughly), you need to go one more to the right then go back to the original pattern, and every 8 movements (roughly) you need to skip a letter. I’m not sure when you’re supposed to exactly make these twists, I basically did them every time I needed something readable while solving the puzzle. See the following grid:
Therefore, the message is
THIS IS MY FIRST PUZZLE ON THE PZLNG STACK XCHANGE SO I HOPE YOU ENJOY DA PUZZ BY THE WAY TE ERRORS ARE INTENDED
FLJX
, i.e. their username. $\endgroup$Y
, and R6C3 and R7C3 should beK
andR
respectively? $\endgroup$