First acquire an original gnome. Mine are 2nd hand and needed cleaning and repairing. Car body filler works well on concrete and plaster.
Next buy latex liquid rubber. It’s cheaper in 5l containers but it’s best to use it up within 12 months. Also buy several cheap synthetic brushes about 20mm.
Rub a thin layer of soft soap onto the gnome. Now spend the next week coating it with 3 layers per day of latex. The brush should wash out with water. Including a wooden base makes the gnome more stable and assists the pouring stage. Let the latex cure for a week.
Larger gnomes, over 25cm or 10 inches, or those with difficult projections, should be made with less layers of latex and then have a 2 part paper mache outer case made while the latex mould is still on the original. Use soft soap again as a releaser.
Now make a pouring support out ouf 2 strips of plywood. It can take a while to get a close fit.
Tip, build up the base of the mould with latex. This stops the weight of mortar pulling the mould through the frame. Mortar made with 1 part cement and 2 parts play sand is strong. Add enough water so that it just pours. Vibrate very well. Cure for 2 days before unpeeling then dry for a week before painting.
These gnomes have been primed with 50/50 pva and water plus a little acrylic colour. Next paint with good acrylic colours and finally spray with car clear acrylic gel.