The reading order gets a bit tangled because there's the main 'Rise of the Evil Sword God' webnovel and some spin-off side stories, but the core volumes are pretty straightforward if you stick to the main serialization. You'd start with Volume 1, obviously, and just go in numerical order through Volume 8, which is where the official translation seems to have stopped for now. Some sites list chapters continuously without splitting them into volumes, but the volume breaks usually align with major arc conclusions, so following them makes the pacing feel right.
Now, there's also 'Evil Sword God: The Early Years' which is a prequel. I'd actually recommend reading that after the first three main volumes, because it fleshes out the protagonist's backstory in a way that hits harder once you're already invested in his journey. Jumping into the prequel first might spoil some of the mystery around his initial grim demeanor. The main story's later arcs, especially the Heavenly Tribulation Arc, reference events from the prequel, so having that context before Volume 6 is useful.