The Laced Running Stitch makes a great wavy line! Use it for embroidering wavy objects or for patterns.
Step 1: Thread Your Needle
Thread your needle and tie off one end.
Pull the untied end of the thread to within a couple inches of the tied off end. This will keep your thread short while stitching and help prevent tangling. As you go along, pull the needle to shorten the untied off end so it continues to pull all the way through the fabric. This gives you twice the length of thread to use while keeping the length short while sewing.
Step 2: Running Stitch
Start with a running stitch.
Start by pushing your needle in from the back of your fabric.
Thread your needle back down.
Continue to go up and down through your fabric and end with your last stitch going down.
Step 3: Lacing
Again, start on the back of your fabric.
Weave the needle under your first running stitch from the right.
Weave under the second running stitch from the left.
Continue to weave left and right of each running stitch.
At the end of your line, thread your needle down to the back.
Step 4: Laced Running Stitch
Here, I used two different colors to create a pattern on the shirt of my embroidered portrait. You can also use the same color for a more solid wavy line.