I think the idea of polygon of forces is that you draw the forces such that at the end of the first force, you draw the 2nd force, at the end of the 2nd force, you draw the 3rd force, at the end of the 3rd force you draw the 4th force, such that all the forces are represented by black lines with arrow heads in the direction of the force, whose length is their magnture etc, and once you've done that with all the forces, if the end of your final force comes to the same place as the start of your first force, then the forces are all in equilibrium.

There are 4 forces involved in this problem:

1) The component of the weight acting parallel to the slope.

2) The component of the weight acting perpendicular to the slope.

3) The friction, acting parallel to the slope.

4) The reaction force of the slope on the block.

I assume you're capable of calculating the magnitudes of these forces using trigonometry and the formulas for friction, etc.

So, once you've found the magnitudes, draw the first force with its length representing the magnitude that you've found, and make sure it points in the right direction. Where that force stops, start drawing the 2nd force. If, when you've done this, the end point of the final force coincides with the start point of the first force, then the forces are in equilibrium.