I love the idea of recycling brown paper bags from the grocery store. It seems so - green - and the price is right ! There are many tutorials on YouTube and you'll find many blogs with tutorials. I used 
this example from Judy Wise. 
The pages have a unique texture and the end result does not need to be perfect. It's a great way to try out new techniques for very little investment and it really does not matter if you mess something up.
Here are a few of my pages - none are complete but it might give you an idea of the end result. I hope you try to make your own paper bag journal. I would love to see it if you do.
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| This is my messy work space. We have a screened porch so I often work there when the weather is nice.
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| I used ink to add color to the scraps left over after ripping the pages to size.
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| This cover is not finished. It is parchment paper glued to the  brown paper and stamped /painted
 I like the way this came out
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| I painted a base coat then added leaves on the left. I am unsure how I will finish the right side.
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| I love the soft colors on these pages | 
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| Trader Joe's bags are heavily printed. I either paint over the design or incorporate it into what I am doing.
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| Example of plain paper - love the texture |