I just found this document http://www.math.toronto.edu/~drorbn/classes/0405/Topology/CoveringSpaces/CoveringSpaces.pdf in which it is said that we can use the classification of covering spaces theorem :
"Theorem: If $B$ is connected and locally connected with base point $b$ and fundamental group $G=\pi_1(B; b)$, then the map which assigns to every covering $p:X\rightarrow B$ its fiber $p^{-1}(b)$ over the basepoint $b$ induces a functor $\mathcal F$ from the category $C(B)$ of coverings of $B$ to the category $S(G)$ of $G$-sets — sets with a right $G$-action and set maps that respect the $G$ action. If in addition if $B$ is semi-locally simply connected then the functor $\mathcal F$ is an equivalence of categories. (In fact, this is iff)"
to show that "Corollary 8: If $B$ is semi-locally simply connected, then for every subgroup $H$ of $\pi_1(B)$ we can found a connected covering $p:X\rightarrow B$ such as $p_*(\pi_1(X))=H$."
But I don't know how to use this theorem (the equivalence of categories) to get the corollary. Could you give me some advices ? Thanks