Action figures exist for all kinds of comic book, movie and video game characters. However, there are very few action figures depicting medieval knights. To create your own, borrow techniques from action figure customizers. With some base figures, extra parts, sculpting, painting and a good bit of creativity you can have figures of knights from a variety of eras, wars and regions of the world. They can be as detailed or as stylised as you like.
Find pictures of the knight you want to create. Encyclopedias, medieval and military history books and websites are good places to find images upon which to base your action figure.
Select a base figure. Action figure customizers start with an existing action figure as a base for their projects. It need not be a figure of a knight. Simply choose a figure that will lend itself well to the figure you want to create. Figures from fantasy movie or video game franchises often come with chain mail, armour and helmets, and they lend themselves well to knight figures. Some superhero figures with suits that resemble scales can double as knights in chain mail.
Check the scale of the base figure you have chosen. Action figures usually come in one of two sizes -- either 6 inches tall or 3 inches tall. You will want to make sure that all armour and equipment, such as shields and weapons, match the scale of the figure. Scale is also important if you swap heads or limbs from one figure to another.
Collect or create armour. Take bits of armour and helmets from other figures, or make them yourself. Epoxy putty or sculpting compound, rolled thin enough to match the scale of your figure and cut to shape, can make good looking armour. When dry, you can glue these pieces onto the base figure with super glue.
Gather weapons or make them from scratch. Many fantasy or movie action figures come with a variety of weapons, such as maces, swords and axes, as well as shields. To make weapons, pieces of sprue from plastic model kits work well for the handles of swords. You can cut the blades from sheet styrene. For spears, use a long piece of sprue dowel and sculpt the tip. For shields, use layers of sheet styrene, cut into different shapes. Golf tees make effective lances. Use a craft drill or sandpaper to put nicks in the weapons.
Paint the figure to match your design. Wash it in warm, soapy water to remove dirt and oils. (If you are attaching sculpted armour, wash the base first.) For realistic figures with a dirty, grimy look, give the figure an overall coat of black acrylic paint. For brightly coloured figures, apply a base of grey primer and an overall coat of white acrylic paint. Paint the figures with several light coats of acrylic paint until you achieve the look you want. Highlight rivets on armour with pencil graphite applied with a finger tip. When finished, seal the figure with a matt spray sealant.
Second-hand stores that take donations of used toys are good places to look for base figures and accessories.
Wear a filter-type respirator when you are spraying sealant on the figure, or use it in a well-ventilated area or outside.