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Lay out your flags: I am using 20 flags that are 2 inches by 4 inches arranged into five columns and four rows.
Cut your paper to the correct height: my book has four rows and each flag is 2" tall, so I'll trim my paper to 8-1/4 inches tall, which leaves a bit of breathing room. We will trim the length later.
Then, fold your thick paper in half length-wise.
Measure along the top and bottom, starting at your fold and going out. I used increments of 1/2 inch, but you can use any measurement you want.
Because my book has five columns, I need to make marks for 11 folds. Use the formula below to determine how many marks you will need for your book: Marks = (# of rows x 2) + 1
Use a ruler lined up to your marks to score the paper evenly, this will make it easier to fold later.
Using the scored lines as a guide, accordion fold the seams and smooth over with a bone folder.
Now you are ready to add your flags! Apply a thin strip of glue to the short edge...
Insert your first flag along the outer edge of the first accordion fold.
Add glue to your next flag...
Insert your second flag just above the first, but on the inside of the accordion fold.
Continue adding flags in the first row, alternating their position inside and outside along the fold.
Continue adding flags working one row at a time, remembering to alternate inside and outside the fold when you glue.
Press your spine down and add pressure to flatten your book into shape.
If your book cover is too wide, you can trim it now using the width of the flags as a guide.
Congratulations! Your flag book is now complete!