1.) Find an expansion slot ideal for your video card. The slot must be the correct type, and it should be as far as possible from other hardware in the system. 

2.) Remove the case insert that corresponds to the slot on the motherboard. This is usually done by unscrewing, but some cases have punch out inserts. 

3.) Insert the video card in the slot. You might need to rock the card in, inserting one end first, then rocking the rest of the pins into place. The old ISA cards may be tougher because of their length. You might not be able to rock them. When pushing down, make sure the motherboard does not flex. If the board tends to bend, it may be necessary to place one hand underneath the board to hold it up. 

Also, in some cases, you may have a problem with the leading edge of the video card's metal plate hitting the case behind the motherboard. The result is that it keeps you from being able to push the card in all the way. I've tried all sorts of weird crap to fix this problem, including taking pliers to the card and actually bending it. Sometimes, you can grab a flat-head screwdriver and pry the hole wider that the card's lip protrudes into. 

4.) Screw the card into place. 

5.) Double-check your work.