If your chart is in an image format (jpg, bmp, png) you can just open it with MS Paint and you are good to go. If your chart is in pdf or other format you will probably need to take a screenshot (use PrtSc) and then paste it onto MS Paint.
Once you have the image of the chart on MS Paint:
1. Under the Brush tab choose the crayon or another semi transparent "brush"
2. Pick a suitable line thickness (For the photo below I chose the thinnest)
3. Pick a colour
4. Mark the areas you have stitched
5. When you finish your stitching session make sure you save your changes.
This free pattern is from http://www.needlework-tips-and-techniques.com/free-cross-stitch-patterns-elephants.html