First put rulers and grid lines on the doc. 'VIEW' 'RULERS' and 'GRID'
Thing I would suggest is to draw a square, how big you want each square to be (say 10mm for example)
Then go to 'OBJECT' 'TRANSFORM' and 'MOVE'. You will be greeted by the 'move' window.
Click on the horizontal box and type 10mm (example only) Then click copy, this will copy the square to 10mm across, leaving your original one there. Next just press CMD or CTRL D, this will repeat the move,so do it as many times as you need squares.
Next highlight the large line of squares you have and do the exact same thing... OBJECT TRANSFORM MOVE, but this time put 10mm in the vertical option box. and click copy. Then use CMD or CTRL D to repeat the move.
If you get stuck let me know the dimensions of the doc and ill knock it up in a minute and email you it.