A single cylinder, double acting, steam engine consists of several major elements:
(1)  The cylinder, where the actual change of heat into power takes place.
(2)  The crosshead, which allows the back and forth motion of the piston to be altered into the rotary motion of the crank.
(3)  The crank and valve eccentric which govern the motion and position of the slide valve controlling the boiler steam into the cylinder and the exhaust steam from the cylinder.
The cylinder components are seen through the open steam chest and in cross section in Figure One (1).

1. Top steam port         2. Bottom steam port   
3. Slide Valve                4. Exhaust port 
5. Crank position          6. Eccentric position 7. Crankshaft
A Steam release on piston, down stroke begins.
B Steam lead begins to lower face of piston.
C. Steam cut-off on piston up stroke.
D. All ports closed to give compression very near stroke end.
If the width of the cylinder port openings and the thickness of the valve ends are the same, the valve will open one steam port to steam and the other to exhaust for even the slightest movement of the valve from a central position.  This does not allow expansive working.  To overcome this, the width of the valve edge is made greater than the steam port width.  This extra width is known as an outside lap (or, steam) lap and inside (or, exhaust) lap.
The admission of steam to either end of the cylinder then only begins when the valve has moved a distance equal to the outside lap and exhaust from the other end of the cylinder begins when the movement is greater than the inside lap and continues until the valve has again returned to this position.  By this means steam is cut off before the full stroke allowing it to work by expansion and is not exhausted until towards the end of the stroke.  Ideally the exhaust is closed just before the end of the stroke to provide a cushion of steam against which the piston can slow down and reverse direction.
The slide valve is moved by an eccentric disc attached to the crankshaft.  A strap around this disc is connected to the slide valve by the slide rod having a knuckle in it.  Simply: the maximum throw of the eccentric is a quarter of a revolution ahead of the crank to put the valve in mid position when the piston is at the end of the stroke.
In practice, it is desirable for the steam to have free entry to the steam port and then to the cylinder.  The port, therefore, has to be opened slightly by the time the piston stroke begins.  To this end, the eccentric is set slightly ahead of a 90 degree position.  This is known as the angular advance.  Additionally this angular advance provides the "cushion" noted above.
An engine as described above running slowly may stop at times when the inconing steam and exhaust act simultaneously.  For this reason, steam engines are fitted with heavy flywheels. to assist them over this point

Slide or "D" valve.  At left, the cylinder as seen from the slide valve chest.  At right, a section through valve and cylinder.  The valve is at mid stroke.
1  Steam inlet to top of cylinder
2  Steam in steam chest
3  Position of piston at top end of stroke
4  Exhaust port
5  Exhaust
6  Slide valve
7  Slide rod (from eccentric on engine shaft)
8  Position of piston at bottom end of stroke
9  Steam to bottom of cylinder

The steam chest is charged with steam from the boiler, its flow controlled by the throttle valve.  A  "D" (slide) valve introduces steam into the cylinder alternately through the steam ports (1 and 9) at the top and bottom of the cylinder, pushing the piston back and forth.  Approximately, steam is exhausted from one side of the piston while steam is introduced on the other.
While this is the basic action, some refinement is required to keep the piston moving throughout a complete revolution of the crank.  First, an eccentric is attached to the crank which is connected to the slide valve via the slide rod (7), causing it to move back and forth over the intake and exhaust ports of the cylinder.
Refer now to Figure Two (2) showing the slide valve position in relation to the crank and eccentric positions, and the resulting piston movement.