You'll need canvas or nylon strapping, a D-ring, a pilot clip and fabric.

Measure how long the collar should be and add app. 5-6 cm.

Cut out a strip of fabric of 2 times the width of the strapping + seam allowance.

Put the strip facing down on the strapping app. 4-5 mm from the edge.

(Alternatively: cut a strip as wide as the strapping. Sew it on the strapping. Sew bias tape on both sides and continue with step 9).

Sew the fabric on the strapping.

Fold the fabric back like in the picture.

Fold a hem...

... and fold it on the strapping.

Sew it.

Topstitch the other side.

Push the collar through one side of the pilot clip and sew as close to the clip as possible.

Slide the D-ring on the collar and sew in place as close to the clip as possible.

Take the other part of the pilot clip and sew in place on the other side.

Cut off excess material on both sides and use a lighter or match to meld the edges of the strapping to avoid fringing.

Done!

(Guus was in no mood to pose though, tired from chasing a rabbit)