The best way to remove highlight pens from fabric is to use rubbing alcohol on highlighter ink. You can spry and dab some solvent on the stain and blot it with a clean white cloth. Make sure to stay gentle on the cloth and do not rub the solvent on the cloth. Otherwise, it will damage the fibres and can also tear the fabric. After removing the stain, follow standard washing instructions with cold water. Repeat the process if the stain is still there, or you can try ink and stain remover on highlighter ink.