NB - I'm not describing in detail how to construct a bargello quilt - if you haven't made one, there are tutorials and patterns available online, and I would suggest trying the technique first before designing your own.
The graph paper I have is four squares to an inch, which is a handy size for quilters, and as the pattern is symmetrical I only need to draw half. I am going to create the quilt from 2 1/2" strips and would like the shorter edge to be 48" so I need 24 rows and I have numbered them on the left.
The first column is the centre strip, so I have labelled it C to remind me I only need to make one and the last column is marked Bg for background - this will be a plain strip of the background fabric at each end. The rest of the columns are numbered from 2 to 18. Each column has also been marked with the width that the fabric must be cut - starting at 1 1/2" at the left and increasing in stages to 2 1/2".
I simplified the design slightly to use 8 shades of two colours A (shown as red) and B (shown as yellow) plus a dark background D, so now it is a case of adding up how much of each fabric to buy.
Let's start with A and proceed step by step,
1) Add up the widths of the colour A columns.
1.5 + 1.75 + 1.75 + 2 + 2 + 2.25 +2.25 + 2.5 + 2.5 = 18.5
2) Note that each of these columns has 16 rows coloured in A but I am only using 8 fabrics so each will appear twice in every column and I need to double the result from step 1
18.5 * 2 = 37
3) Remember the diagram is only half the design, so I need to double again
37 * 2 = 74
4) Finally, column C is the centre and only needed to be included once, so I can deduct it from the total.
74 - 1.5 = 72.5
Cutting across the width of fabric will give strips that are around 42" long, so this tells me I need two wof strips that are 2 1/2" wide for each shade. So I want to buy 5" (this is where it gets a bit messy as fabric is sold in metric measurements in the UK, the nearest round number is 20cm).
Repeat steps 1 to 3 for colour B.
Calculating the background fabric is slightly different. I want to cut the two Bg columns down the length of fabric so I know I need a piece of fabric that is 48.5" long, I am just checking that I can get all the other strips from this piece as well, and I would like to cut them across the width. Assuming the fabric is 42" wide and I need 5" for the two Bg strips, I have a piece left that is 37" wide by 48 1/2" long.
5) Every other column has 8 squares coloured in D, and in most cases there are some at the top and some at the bottom. The finished height of each row is 2" and if we add 1" to each column for seam allowances we have
2 * 8 + 1 = 17.
6) Remembering that the diagram is only half the design, I double this
17 * 2 = 34
And I can see that this will easily fit across the width of the fabric.
7) Adding up the width of the columns from C to 18 gives 37" and this will easily fit down the length of the fabric.
So converting to metric, I will need to buy 1.25m of D to complete the quilt top (backing and binding to be decided later).
This is as far as I'm going with describing the design process, but I have one final tip to share for assembling the quilt. I sewed together the strip sets for colours A and B (which consisted of 8 shades running from light to dark x 2) and before going any further, wrote out a set of labels for the whole quilt on scraps of paper. I used the headings from the diagram and added a prefix L or R for left/right side - the exception is column C as it marks the centre. Odd numbers and C are for colour A, even numbers for B. I found it easier to take each colour and cut all the strips of the same width at once eg all the 2" then all the 1 3/4" and so on, pinning on the labels as I went.
I repeated the process for the background fabric, however this time there are either one or two pieces per label.
The quilt then comes together easily in two stages.
1) Attach the background pieces to the strips of A and B by matching the labels. Refer to the diagram to see which of the pair of background strips goes at the top (lighter end of A or B) and which at the bottom (darker end) when they are different lengths. Keep a label on each column.
2) Join the completed columns in number order - I joined them all into pairs, then the pairs into fours and so on.