I found Linked double chain stitch in an old embroidery book called The Batsford encyclopaedia of embroidery stitches by Anne Butler. If you ever find this book in an secondhand store it wi well worth picking it up if priced reasonably.
Linked Double Chain is an interesting variety of chain stitch. If you are a beginner you might like to make sure you are familiar with chain stitch and with twisted chain stitch since Linked double chain consists of two chain stitches worked side by side which are then linked with a twisted chain. This combination is worked to form a line.
To work the stitch start with a single very open chain stitch. It is a bit sloppy looking as you can see.
Next work a second chain next to the first. Make the stitch so that the two stitches are side by side.
At this stage the y look a little like buttonhole stitches.
Bringing the point of the needle out a short space down the line to be stitched. Cross the thread over the needle, then wrap the thread under the needle point.
Pull the needle through the fabric.
What you are doing is working a twisted chain stitch into the base of the first two stitches.
If you are unfamilar with twisted chain stitch you will find the directions here
At this point it should look like this.
Next work two chain stitches side by side emerging from the base of the Twisted Chain.
Continue along the line in this manner.
This is a marvellous stitch as it will follow a curve well and lends itself to all sorts of interesting experimentation.
You can widen the chain stitches and play with spacing
It looks particularly effective if you use a thread with a firm twist.
Silk ribbon also works well with this stitch. It lends itself to being embellished with beads too.
I hope you enjoy experimenting with this stitch.