Identify the initial amount, (N_0)
Identify the remaining amount after time (t), (N)
Use the decay formula (N = N_0 left(frac{1}{2}right)^{t/t_{1/2}})
Rearrange to solve for half-life:
(t_{1/2} = dfrac{t ln 2}{ln(N_0/N)})
If the amount halves in a known time, that time is the half-life
If the decay constant (lambda) is known, use (t_{1/2} = dfrac{ln 2}{lambda})