Take train platform 2 to Nuevos Ministerios station (Third station; 16 minutes trip). Timetable:  http://www.renfe.com/EN/viajeros/cercanias/madrid/index.html

In Nuevos Ministerios take metro line 6 direction "Cuatro Caminos" and get out in "Ciudad Universitaria" station (Fourth station; 12 minutes trip). See metro map below ("PLANO-METRO-MADRID-2019.pdf") 

Walk to CIEMAT (16 minutes trip). See walking route below ("to_ciemat_f.pdf")