How to do \mathbf in handwriting?

Solution 1:

Since it is hopeless to write bold-face characters by hand, people introduced symbols like $$ \vec{w}, \quad \underline{w}, $$ for vectors. Matrices are usually written in roman characters, but with upper case: $A$, $B$, $W$, etc.

Mathematicians do not like strange symbols for vectors, and they tend to write simply $v$, $w$ as if they were writing scalar quantities. The context makes the difference. Physicists and engineers prefer arrows, as far as I know.

Solution 2:

Why not use a tilde & a harpoon over the letter to distinguish between a matrix & a vector respectively? In control systems, capital letters are used to denote the Laplace Transform of a time-domain variable & since multivariable systems are not uncommon this could distinguish that.