How do I put tranny fluild in my 1998 Chevy S10

The transmission fluid is added through the dipstick tube

Yes, you will add it through the hole in the side of the MANUAL transmission once the plug is removed. You may need a ratchet with an extension to remove the plug. Then you will have to devise a method for getting the fluid into the transmission which can be very tricky. I use the plastic container the fluid comes in plus a nipple that comes on a bottle of rear end lubricant and a section of clear plastic hose that is available at a hardware store. Then you can squeeze the bottle to get the fluid into the hole. When the fluid oozes back out its full. Quickly screw the plug back in place! Its messy and not convenient but that's the way GM designed it.

The filler plug is on the driver's side of the tranny. It looks exactly like the drain plug on the bottm of the tranny.

Automatics use the dipstick tube, manuals do not have a dipstick.