HSK form machine readability
Using 4B pencil, mark the whole box with solid black. (For me, 2B is not dark enough and 4B+ is too soft, green 中华牌铅笔 is the best.)

Sharp your pencil with a knife (don't use pencil sharpener) and make sure the pencil top can cover the whole box with a single stroke, this can save your time. 

Prepare 2 or more pencil for the test and protect your pencil top, also prepare a good eraser. 

Print out or buy sample answer sheet and do some practice. 

Normally this sheet will give to you before the real test paper, immediately mark your personal information at this stage will save some time and double check your information is correct. 

Here is a Chinese Version Guide