We first saw the 3-digit symbol and knew it was 100. Then realised that the big diamond represented 1 and the small diamond represented 0. Next, we matched up the digits that used the same symbol twice and numbers with 1 digit. These were:
11, 22, 33, 44, 55 etc. 1, 2, 3, 4, 5, etc.
After, we went down the columns because we knew that 4 was a coloured in circle, 9 was a square etc. We knew most of the combinations because once we found out what each symbol was, we managed to figure out the rest. Also, another difficulty was all the puzzle pieces had to match up but, when we knew all the combinations the pieces just slotted into place.
Top Tips to get you started:
Always start with the 3-digit symbol(100)
Cut them all out at the same time (not literally) so you've got them all to play with