Take steps to ascertain that your student is left-handed, if you aren't positive of the dominant hand. Play catch with a small ball and pay attention to which hand the child throws with. Another idea is to offer the child a few uncapped jars and ask the child to screw the lids on. The hand they handle the lids with is their dominant hand.
Instruct student to hold the pencil between their left thumb and forefinger, about an inch from the point. Left-handed students should learn to write cursive before manuscript if possible, which will help them avoid the bad habit of hooking their hand around to write.
Instruct students to keep their paper tilted clockwise as they work, which forces left-handed students to start their letters properly from the bottom to the top. Starting from the top and working to the bottom for each letter is a bad habit picked up when left-handed students tilt their papers counter-clockwise.
Mark papers to remind left-handed students to work from left to right, since it may at first feel more natural to them to write from right to left.