Spanish phrase
Voy a rellenar ~.
Meaning
I am going to fill out ___.
When to use it
States the speaker will fill out a document. Common at borders and official counters, neutral and straightforward.
Example dialogue
Voy a rellenar la declaración de aduanas.
I am going to fill out the customs declaration.
Aquí tiene el formulario y un bolígrafo.
Here is the form and a pen.