The Archers Word
Brace - action
The act of attaching a bowstring to a bow such that it is ready to shoot.
There are several ways to attach a bowstring to a bow, usually depending on the bow that you intend to brace.
Foot bracing technique:
A simple method for weaker bows in which one of the limbs is held down with the foot, while bending the bow to such compressive state that one can attach the bowstring from limb to limb. This method is though the least recommended one, since it often damages the limbs after many repetitions.
Step-through technique:
A method in which the archer steps with one foot in between the bowstring and the bow, and uses both the hand on one limb and the other foot for the other limb to press the bow to a compressive state, enough to attach or loosen the bowstring. This method is much more recommandable than the foot bracing technique and is widely used for many bows such as the english longbow and the reflex bow.
Stringer technique:
For recurve bows or similar, there is another method, in which the archer takes a string to press the bow to its compressive state. On one side of the string, there is a small bag, which is attached to one limb, while on the other side of the string, there is a loop, to put around the opposite limb. By pulling on the bow and string into opposite directions, one bends the bow enough so to attach the bow string.
Bow press:
More complicated bows such as the compound bow, which uses cams or pullyes, are pressed into compressive state by a bow press, which allows attaching the bowstring to the bow.

Useful References:
Livestrong, "How to brace a bow":