Just suppose you have a time grid:
t1-t0, t2-t1, t3-t2, ..., tn-tn-1 (tn=T)
you want to generate a random motion W on it:
W1-0, W2-W1, W3-W2, ..., Wn-Wn-1 (Wn=W)
and you want the incremental W's to have the same independent distribution with expectation 0 and variance s*s
So you get the total...