Every manifold has an "orientation double cover" $\tilde M \to M$, whose elements are points of $M$ equipped with an orientation at that point. It comes equipped with a rather tautological orientation. If $M$ is connected, $\tilde M$ is connected iff $M$ is non-orientable.

Given a (reasonable; certainly this includes every manifold, CW complex, ...) space $X$ there is a bijection between isomorphism classes of real line bundles, equipped with a fiberwise metric, and double covers. The forward map is given by passing to the subspace of norm-1 elements; the inverse takes a double cover, trivializes it over an open cover of $X$, and then constructs the real line bundle with the same transition maps $U_i \cap U_j \to \pm 1$.

Now pass from $\tilde M$ to the associated real line bundle $\ell$, considered as a smooth manifold. This has an orientation: the most irritating part of checking this is on the 0 section. Explicitly, $T_{m,0}\ell = T_m M \oplus \ell_m$. Pick a basis $B$ of $T_m M$ and a norm 1 element $o$ of $\ell_m$; say that $(B,o)$ is positively oriented if $o$ says that $B$ is positively oriented.

Explicitly for $\Bbb{RP}^{2n}$, this is the tautological line bundle; its total space can be identified with $\Bbb{RP}^{2n+1} \setminus \{pt\}$, which is indeed orientable.