From an arbitrary element in a number field find linear combination with respect to basis

Question

I have an algebraic number $\alpha$ (a rough numeric approximation) over a number field $\mathbb Q(\theta)$. But I do not know how to express $\alpha$ as a linear combination of $1,\theta,\theta^2,\dots \theta^{n-1}$ (where $n=[\mathbb Q(\theta):\mathbb Q]$). Is there an algorithm how to obtain this linear combination?