A few tips before starting: 

Use clamp type spring compressor rather than a 'pry-bar' type compressor; Have a magnetic pick up tool ready, Have a small flashlight handy, Be sure to fully seat umbrella (exhaust) and positive (intake) seal fully-press evenly on both sides using boxed wrench for example, & Be sure nothing falls in the engine !
 

Some steps are omitted because they are not needed or required to complete this repair.  Examples of this are "Remove the serpentine belt to make rotating the engine easier", "Cleaning the motor as well as possible so debris doesn't fall inside, or; "Inspect all parts & hardware for damage before installing"   Some measure of common sense is expected !!

**  I don't advise using air pressure to keep the valves in place when changing the Valve Stem Seals. If someone steps on the air hose, the air line breaks, the Compressor loses power or the valves are not in the "best" position, then the air pressure could be lost and the valve could fall into the cylinder. Stuffing or filling the the cylinder will insure that the valve stays up - at all times - until the stuffing is removed. The best type of 'stuffing' I've found is simply and old spark plug wire...... It's strong enough to push & pull thru the spark plug holes repeatedly without disintegrating and can not damage the engine internals in any way.


 

How To Remove / Install Valve Stem Seals on a Small Block Chevy V8 Engine

 
 1- Remove all plugs
 2- Rotate Balancer to #1
 3- Push old spark plug wire ( or equivalent ) into #1 Spark Plug hole
 4- Remove rocker arms from #1 Intake and Exhaust valve
 5- Compress Spring from #1 Intake (or Exhaust) valve 
       ( You will probably need to tap or strike the spring compressor or 
       spring assembly 'down' with a small hammer or similar tool in order to
       separate the assembly from the valve stem and free the 2 "keepers" )
 6- Remove keepers ( Magnetic tool is a good idea ! ')
 7- PULL spring assembly off ( Probably NOT going to just 'slide' off !)
 8- Remove and clean any debris from old stem seal
 9- Pry Intake positive seal off ( or exhaust umbrella )
10- Install positive seal( or exhaust umbrella )
11- Slip spring assembly back over the valve stem
12- Slip stem seal over valve stem ( it sits inside the bottom groove )
13- Install keepers ( Pull spring assembly up to hold keepers in top groove  )
14- Remove the compressor
15- Install the rocker ( no adjustment at this point - do them ALL later )        
16- Move to the Exhaust or Intake valve for same cylinder and repeat steps 5-16
17- Remove Spark Plug wire from Spark Plug hole 

Once cylinder is complete:

18- Rotate engine 1/4 turn, push spark plug wire into #8 cylinder spark plug
       hole & repeat steps 4-17.
19- Rotate engine 1/4 turn, push spark plug wire into #4 cylinder spark plug
       hole & repeat steps 4-17.
20- Rotate engine 1/4 turn, push spark plug wire into #3 cylinder spark plug
       hole & repeat steps 4-17.

Continue until all seals are replaced ( thru the 1-8-4-3-6-5-7-2 firing order)

21- Once the engine is rotated back to # 1 with all seals installed and rocker arms temp installed,.... follow the steps show here to set valve lash and pre-load lifters ( Or after Installing a set of Rocker Arms )

Small Block Chevy Valve Lash /Rocker Arm Adjustment