Avanza bus stop location : where in Madrid airport

TZ,
Most International flights arrive at Terminal 1 and Terminal 4. You may want to contact the airline you'll be flying to find out this information.

You can take the Avanza Bus from either of these terminals ( 1 or 4).
You can find the location of the Avanza Bus at either of these two terminals here on the Barajas Airport website; just click on the appropriate terminal link in order to see a diagram/ map of bus stop location.

If you need transport between terminals, you can take the free shuttle bus provided by the airport.

Have a wonderful trip!

Edited to add...
Please note that the Avanza Bus originates in Terminal 1 (bus parking lot), then it stops at Terminal 4 ( about 15 minutes later) outside of arrivals area (ground floor).

According to the Avanzabus website, it looks like you need to specify from which terminal you'll
be boarding the bus.

It's also recommended to buy your bus tickets in advance (buses fill up), however, keep in mind that if you miss your bus, you'll need to buy a new ticket for the next bus.
Bus tickets can be changed up to 2 hours prior to departure, so you'll need to take this into consideration when booking your tickets.

Just follow the signs.

Thanks for the help !

You're welcome TZ!