best way to get from spain to portugual

You are correct, a bus.

There are very poor rail links between the two countries due to traditional hostility and the very sparse population of most of the border regions. Consider bus or flying.

Another option is to rent a car in Andalucia, do a loop and return to Spain.

Buses run directly from Seville to Lagos and a few other stops in Portugal; see www.damas-sa.es. There's no point in trying to mix in a train on this route. I don't think you'll find direct buses from other cities like Cadiz or Jerez; you probably still connect at Sevilla.