Instruction for filling out an answer form

I am designing an answer form for multiple choice and true-false examinations. The form has also an instruction how to fill it out. I would like to know if the English is correct and if it is clear what I mean. The students have to fill in the box of their choice for every question, that is to “blacken” the box as they say.

Here is the instruction as I formulated it:

INSTRUCTION TO FILL OUT THE FORM

1. Use a blue or black ballpoint for filling out the requested information at the top of the form and for encoding your student number in the designated boxes.

2. Use a pencil (preferably HB) when giving the answers. Use an eraser for corrections. Do not use correction fluid or tape.

3. Answer every question by filling in the box of your choice (fill in one box only!).

At first I wrote regarding point 2 “Use a pencil (preferably HB) for filling out the answers.”, but someone told me that “when giving the answers” would be better English. Further I would like to know what the correct place of “only” is. Should one write “fill in one box only!” of “fill in only one box”?

I would appreciate your comments. Thanking you in advance.