Heavy chain stitch is one of the many variations of chain stitch. It produces a solid heavy stitch, a bit like a braid which is perfect for outlining densely stitched shapes that need the definition. It is worked in the opposite direction from regular chain stitch.
Begin by bringing your needle up at the start of the line you wish to work. Take a small stitch about 2mm down along the line.
Bring your needle up along the line, 2mm below your previous stitch.
Using the eye of your needle or a tapestry needle, take it underneath the first stitch you made. Be careful not to catch any fibres from the first stitch.
Take your needle back down into the same hole that you brought your needle up in.
Bring your needle up about 2mm below the stitch you have just created. Take your needle through the very small stitch you first made. You should now have two stitches going through that first stitch.
Again take your needle down in the same hole that you brought your needle up in.
Bring your needle up again, on the line 2mm below your previous stitch. Take your needle through the stitch you created in step 6.
Make sure you do not take your needle through your previous stitch as shown in the image below, otherwise you will end up with a normal looking chain stitch.
Continue to work your stitches following the above step. Remember to always take your needle through the second from last stitch that you worked.
Once you reach the end of the line you can take the stitch down in the same way as before and finish your thread.
And here is what your completed heavy chain stitch should look like.
I hope this post helps and you found the information useful. As always, please feel free to comment with any questions!