How to set power when given fixed supply voltage?
You're almost there:

\$P = V^2/R\$

so

\$R = V^2/P\$

so for a given V and P you just choose R to give the required power.

E.g. if you have 12 V and you want to dissipate 60W then plug those numbers in and get \$R = 2.4 \Omega\$.

A possibly simpler way to look at this is to break it down into two steps:

(1) for a given V and P you can calculate the required current, \$I = P / V\$

(2) when you know the current, I, you can calculate the required load resistance as \$R = V / I\$

So using the above example, you'd get:

\$I = P / V = 60 / 12 = 5 A\$

\$R = V / I = 12 / 5 = 2.4 \Omega\$