  Recent numerical studies of the $J_1$-$J_2$ model on a square lattice suggest
a possible continuous phase transition between the N\'eel state and a gapped
spin-liquid state with Z$_2$ topological order. We show that such a phase
transition can be realized through two steps: First bring the N\'eel state to
the U(1) deconfined quantum critical point, which has been studied in the
context of N\'eel -- valence bond solid (VBS) state phase transition. Then
condense the spinon pair -- skyrmion/antiskyrmion bound state, which carries
both gauge charge and flux of the U(1) gauge field emerging at the deconfined
quantum critical point. We also propose a Schwinger boson projective wave
function to realize such a Z$_2$ spin liquid state and find that it has a
relatively low variational energy ($-0.4893J_1$/site) for the $J_1$-$J_2$ model
at $J_2=0.5J_1$. The spin liquid state we obtain breaks the fourfold rotational
symmetry of the square lattice and therefore is a nematic spin liquid state.
This direct continuous phase transition from the N\'eel state to a spin liquid
state may be realized in the $J_1$-$J_2$ model, or the anisotropic
$J_{1x}$-$J_{1y}$-$J_2$ model.
