What you need4 x 6 inch (10.5 cm x 15cm) cotton fabric or fabric of your choice.
1 yard (1 mt) of cord, ribbon or binding.
The cloth cover protects your keys from grime as well as hides away the obvious clanging clutter of keys around your neck.
Take the fabric piece and fold it in half lengthwise (6 inch side) . Turn inside out and sew down the edge to form a tube.
Sew a small hem across the top of the tube. Using a running stitch draw up the top to form a small hole as shown with the blue example below. This is where your cord will thread through.
You can use ribbon for the lanyard or an existing work lanyard.
If you are using a ribbon or cord, consider attaching a press stud for easy release or simply tie a knot around your keys. This will really depend on your requirements. Eg. Do you need to un-attach your keys when using them?
If you are using an existing lanyard then finishing off is the same, simply thread your cord through the hole with your keys at the bottom.