The simple answer to this question is by striking flint and steel. Lighting a fire was a big deal in the middle ages–both incredibly common place and sometimes not that easy to do if the conditions weren’t right. Fires were kept lit in houses all the time and woe to the child who was supposed to be watching it and it went out 🙂

Lots about medieval fire lighting here:  http://www.sthubertsrangers.org/making_fire.htm

Even more here:  “Since matches did not become available until the mid-1800’s, prior to that time people had to make fires in other ways. The two most common methods of fire-making before the advent of matches were friction and percussion.”  http://www.vikinganswerlady.com/fire.shtml

Four medieval firesteels.

“A fire striker (or fire steel) is a piece of high carbon or alloyed steel from which sparks are struck by the sharp edge of chert or similar rock. Modern fire strikers or artificial flints consist of ferrocerium alloys.

From the Iron Age forward and prior to the invention of the friction match, the use of natural flint and steel was one of the most common methods of fire lighting.

More recently the term “fire striker” has become one of the names used for artificial flints, metal rods of varying size composed of ferrocerium, an alloy of iron and mischmetal (itself an alloy primarily of cerium) that can generate sparks when scraped with a sharp, hard edge. Iron is added to improve the strength of the rods. Ferrocerium is also used for the “flints” used in cigarette lighters.

When natural flint and steel were commonly used, the fire steel was often kept in a metal tinderbox together with flint and tinder.” http://en.wikipedia.org/wiki/Fire_striker

“The process of starting a fire with flint and steel is straightforward. A spark is created by striking the steel down across the sharp edge of a flint or quartz rock. The rock shears off tiny flakes of metal, creating sparks. To nurture the spark into a flame, the spark is caught on a highly flammable material called char cloth (‘charcoal cloth’). Once the char cloth is glowing, it is transferred to a bundle of tow and blown into flame. The flaming tow is used to ignite successively larger tinder like wood shavings, dry pine needles and small twigs.”  http://www.mainlymedieval.com/store/product_info.php?products_id=2065

Char cloth is made especially to burn easily:  “On camp, a simple way to make ‘charred-cloth’ is to throw a piece of clean linen, cotton or calico on a fire and let it catch alight : remove it safely and either put the fire out on your cloth by placing a plate, dish or a bowl on it (to restrict the air) or put your booted foot on it on dry ground. When the fire is completely out, save the charred edges nearest the edge of the burned cloth and trim them with scissors or your knife as they are ‘char-cloth’ and the charred edges will catch a spark from flint-and-steel.  However – a much more efficient method of making better char-cloth is to slice or tear up your clean cloth into two-inch square pieces and place them in a sealed tin for ‘burning’ on the embers of a campfire.  Use your dagger to pierce a small hole in the tin – as seen, in both ends of the tin (both base and lid). Loosely pack the tin half-full with your torn-up pieces and place the sealed tin on the embers of a campfire. Smoke will soon issue from the holes in both ends of the tin – which will be noxious if it reaches your nose and eyes – but the basic idea is to char the cloth without burning by restricting the access of air.  It doesn’t matter if the tin gets red-hot but you can assist the process by occasionally turning the tin around on the embers. … in medieval experiments, a clay-pot with a lid did produce a similar result.  http://www.sthubertsrangers.org/making_fire.htm

“Before John Walker invented matches in 1826, fire had to be created a different way. The trick was to produce a spark from friction. Friction can be created by striking stones together, drilling or sawing wood against wood, or using a firesteel that rubbed flint with steel.”  http://www.jillwilliamson.com/2010/05/medieval-facts-lighting-part-three-oil-lamps/