A simple DC generator is constructed the same way as an AC generator except that there is one slip ring which is split into two pieces, called a commutator, so the current in the external circuit does not change direction. The layout of a DC generator is shown in the figure below. The split-ring commutator accommodates for the change in direction of the current in the loop, thus creating direct current (DC) current going through the brushes and out to the circuit. The current in the loop does reverse direction but if you look carefully at the 2D image you will see that the section of the split-ring commutator also changes which side of the circuit it is touching. If the current changes direction at the same time that the commutator swaps sides the external circuit will always have current going in the same direction.
The shape of the emf from a DC generator is shown in the figure below. The emf is not steady but is the absolute value of a sine/cosine wave.