If by "create them so they are singular" you mean reducing an 8 character sheet into a 1 character sheet, then --> Crop the character's sheet you want into a 96x128 pixel sheet. (By itself). Then save the file with a "$" in the beginning of the name like this: $charactername.png
From there you can import that file and VX will recognize that it's a sheet that should be processed 3 tiles wide and 4 tiles tall.
Likewise, if you you don't include the '$' at the beginning of the filename, then VX will assume that the file is a character sheet that is 12 tiles wide and 8 tiles tall.
Also, you can use MSPaint, Gimp, or Photoshop to do any of this. MSPaint obviously comes on your computer, Gimp is a free program you can download that's similar to Photoshop and then there's Photoshop which you'll need to buy or pirate.
I attached 2 examples.
Example 1: $GoldArmorGuy.png is what I think you mean.
Example 2: Characters07.png is what you're removing it from and turning into Example 1.