Guatemala City to Valle Nuevo to Juayua

Hello,

I need to travel from Antigua to Juayua in El Salvador. I think the route I should take is:

Antigua-Guatemala City-Valle Nuevo border crossing-Ahuachapan-Juayua

My primary question is how to go from Guatemala City to Ahuachapan.

The LP guidebook says that Melva Internacional goes from G.C. to San Salvador, crossing the border at Valle Nuevo. Would this also go through Ahuachapan and could I get off there to transfer to Juayua? Are there other busses that I could take?

Thank you very much--we leave to tomorrow for our month down south!!!!