In my experience, you'll want to start with the fluid level and condition check. Should be bright red and smell fresh. If it's brown, or smells burnt, change the fluid and see if that helps. If that's ok, then try to figure out what gear(s) it's slipping in. Questions like what gear(s), what temperature(s), and what speeds it does it will be VERY helpful to a transmission repair facility.
answer check fluid first if it is burnt and dirty the damage has already been done used to have adjustments on bands in transmissions they are not built the same anymore either your transmission is wearing out or your torque converter is going out