Making Correct Tension
Your tension should be consistent throughout your work. A loose tension will look lumpy and a tight one will distort the canvas. Sometimes this will not block out properly. To avoid errors, your posture should be consistent. Sometimes sitting straight and at other times sloughing will change your stitching pattern. When you needlepoint, sit in the same position. Back should be straight and legs uncrossed. Good posture will also alleviate back strain and leg fatigue.
Try to stop your stitching when you have finished an area or when you have finished a strand of thread. Stopping in the middle could change your tension making uneven stitches.
In working stitches, pull the thread in a complete movement. Do not pull, stop and then finish pulling. Avoid stitching in a jerky motion. This will create an uneven area.
Having the canvas mounted in a frame will help. Also, you may want to invest in a floor frame.
Is there a way to correct the tension once the stitches are in?
If you find stitches with loose, incorrect tension, turn the project over and gently tug the stitches towards the area in which you have stopped. This technique is great for smaller areas. If the area is large or you have very loose stitches, it may be best to rip out and start again. Nothing will help stitches which are too tight. You will need to get your scissors and start cutting.