I'll step you through it, but it does seem as though everyone is asking this question. Here goes:
Step 1: Get the pokemon you want: [SCYTHER]
SKIP TO STEP TWO IF YOU ALREADY HAVE A L100 SCYTHER
Enter the two codes:
Leave the GS switch off, and start the game.
Load your file (CONTINUE).
Now get set up to fight a level 100 Scyther (bring a master ball).
Turn the GS switch on, then off.
Now you should fight a Scyther. Capture it.
Save the game, and turn off the GB. Turn it back on, and...
Step 2: Now you want to use the Stat EXP codes. You enter just the following codes:
(only enter the codes that match your color cartridge, obviosly).
MHPH 01FE7CD1 01FE7BD1
MHPL 01017DD1 01017CD1
ATTH 01FE7ED1 01FE7DD1
ATTL 01017FD1 01017ED1
DEFH 01FE80D1 01FE7FD1
DEFL 010181D1 010180D1
SPCH 01FE82D1 01FE81D1
SPCL 010183D1 010182D1
SPDH 01FE84D1 01FE83D1
SPDL 010185D1 010184D1
In all cases, the value listed goes into the ??.
Now, keeping the GS switch off, start the game, then load your file (CONTINUE option again). Now put that L100 Scyther into the top spot of your party. Now turn the GS swtich on, then off again. Now put the monster into a box, and bring it back out.
Your Scyther should then be L100. Pokecenter it to max HP, then save the game.
If you are curious as to what to do, please post EXACTLY what you do not understand.
Let me know if this works.
It sure as shit should.