Subtracting vectors combines the ideas of negative vectors and addition.
Remember how a negative vector has same size but opposite direction, well this means that the concept of subtracting a vector is the same as adding its negative.
That is that
Geometrically it looks like this
here are vectors $u$u and $v$v
$u+v$u+v will look like this (remember to head to tail it)
$-v$−v will look like this, (same size as $v$v, just opposite direction)
and hence $u-v$u−v, or $u+-v$u+−v will look like this
Here are both situations to compare on the one diagram
Of course if we use matrix notation, then this becomes a simple process of matrix subtraction.
This applet will allow you to practice making vectors, and then demonstrate their subtraction via a geometric representation.