Work with the ‘purl’ side of the fabric facing you.

Cut a length of yarn which is longer than the line that you are making.

Starting at the bottom, fasten the yarn at the back of the fabric.

Insert the needle through the fabric and pull the yarn through.

Insert the needle under the first horizontal bar.

Pull the yarn through.

Insert the needle under the second horizontal bar and pull the yarn through (skipping the first bar).

Repeat step 7 until you have completed the whole line.

To speed things up, insert the needle under several bars at a time.

Repeat this for every stripe needed, according to your pattern.