Take all the furniture out of the room. Spread tarp over the floor.
Spread drywall joint compound over a section of the drywall, covering an area about 4-by-4 feet, with your drywall knife. Lay the compound on at about a quarter-inch thick.
Use your cement trowel to flatten and shape the joint compound, putting ridges and lines in it and giving it a rough, textured look. Texture the entire area of compound.
Spread joint compound onto the next 4-by-4 area of the wall, next to the first area, again using your drywall knife. Texture the surface of the compound with your cement trowel as before. Make sure the edge of the new area is completely consistent with the edge of the first area, so they don’t look like separate sections.
Repeat, spreading compound over the wall section by section and texturing each section. Assess the look from a slight distance after you complete each section, to ensure consistency. Do the entire wall.
Let the compound dry for 24 hours. Paint if desired.