Before adding any fluid:
* Locate the fill plug: Consult your owner's manual to find the precise location of the fill plug and the type of fluid specified (usually a GL-3 or GL-4 gear oil).
* Check the fluid level: This usually involves finding the fill plug and checking whether fluid comes out when the plug is slightly loosened. The manual should specify the correct checking method. *If* fluid flows freely, your transmission is likely full and adding more is unnecessary and could damage it.
* Use the correct fluid: Using the wrong type of fluid can damage your transmission. Again, your owner's manual is the definitive source for the correct specification.
If you are unsure about how to check the fluid level or add fluid, it's best to take your vehicle to a qualified mechanic. Improperly adding transmission fluid can cause serious damage to your transmission.