Turn on the grid view>page grid
Hold control whilst you use the rectangle tool to draw a small square in the middle of the page (the red 1 in the centre), aligned with the grid.
Select the small red square and duplicate it with control-D
Drag the duplicate square under the first red square and colour it yellow (the yellow 1 exactly under the red one)
Hold control whilst you use the rectangle tool again to draw another square starting from the top right vertex of the red 1, and finishing when it matches the edge length of 1red + 1yellow. Colour it green.
Carry on to add the blue 3, the purple 5, the red 8, the yellow . . . . . . .
The edge length of each of the first two squares is one unit, the edge of the third square is the first two added together so has width of 2 units
The forth square has an edge length of square 2 plus the first 1 , so has an edge length of 3
the next edges are 5 and then then 8, then 13, then 21, then 34 . . . .
Starting with 0 and 1, each new number in the sequence is simply the sum of the two before it.
Using Excel we can show shows how the ratios of the successive numbers in the Fibonacci sequence quickly converge on Phi. After the 40th number in the sequence, the ratio is accurate to 15 decimal places.