Place an arrow in each empty square. Every arrow can go horizontally, vertically or diagonally and points to at least one square with a number. The numbers indicate the total number of arrows that point to them.
The square that first draws our attention is the one with the digit 7 in it. If we count carefully, we see that at most 8 arrows could point to it. Now it happens to be that not both of the arrows in the middle column can point towards the 7, because in that case the two ones would have too many arrows pointing at them. From this it follows that the other 6 have to point to the 7, that exactly one of the arrows in the middle column points to the 7, from which in turn you can conclude nothing else will point to those ones.
If you click on a number with the left mouse button, a dot will occur. If you click more often, more dots will appear. In this way you can mark for yourself how many arrows are already pointing at a specific number. With the right mouse button you can indicate that you are done with a number (a circle will appear around it).
Another tip is that you can draw the arrows more quickly by using the digits 1 through 9 on the right hand side of your keyboard. For example, the 8 gives an arrow pointing up, the 6 gives an arrow pointing to the right and a 9 gives an arrow diagonally pointing to the upper right.
Now consider the 3 in the rightmost column with digits. In principle there are 4 more empty places that are in line with this square, but only the square below this particular column would not also point to a 1 that has been pointed to enough already. So this will become an arrow pointing upwards.
The 3 in the lower right must and can only be pointed at by 2 more arrows. Hereafter it is clear that in the middle column the upper arrow points downwards. The remaining two arrows point to the 4 in the upper left.