If you're unsure when the fluid was last changed, especially the filter, it's best to change it at the same time as doing the fluid change.
What I do is buy a filter kit, it comes with the filter, filter gasket and pan gasket. The transmission pan removal is a bit of a challenge for a first timer, you will need to disconnect the dipstick tube from the bottom of the pan, but you need to be careful you don't wreck the pan as the captive nut which the male dipstick threads into is welded on, and you need to hold it with a spanner whilst undoing the male thread.
Once you have the pan off, prepare to take a bath in ATF, which you will want to have drain for at least a couple of days to minimise making a mess. What I also do is drain the fluid first from the drain bolt, leave it for a day, then once the pan is off leave it for another few hours to let the last of the old fluid drip out.
The filter is held on with 5 or 6 bolts, pretty simple, remove old, replace new
Draining this way gets about 4 litres of fluid out, which is roughly half of the total capacity including what stays in the torque converter, but you can drain that out next time, at which point the old remaining fluid will have mixed with the 4 litres of fresh fluid you added. Clean the pan so it's clean enough to eat out of, fit the new gasket, torque the pan bolts to 5Nm, re-fit the dipstick tube, fill with 4 litres of any ATF (or if you're like me buy a quart of lucas oil transfix which means you will only need 3 litres of ATF) synthetic is better but you don't need it.
After about a month of driving, drain the fluid again, but only from the drain bolt (don't overtighten the drain bolt it strips easily), measure what comes out, then re-fill with the same amount of new ATF, job done