I've made quite a few Bargello ... I have a few tips.
1) Choosing fabric is very important. You will need to consider both color and value. Value is light vs dark. To help you choose value you can either use a color shield (blue for reds and browns, red for all other colors), or take a picture then convert it to B&W. Basically you need to strip the fabric of color so you can see the light from the dark. Also with regard to choosing fabric - avoid large prints with a lot of degree of change in either color or value. For example a large sunflower on a green background is not a good choice. Look for fabrics that have a print smaller than the smallest bargello square you will cut. The end result will be a better blend from one strip to another without the wrong color interrupting the flow. Study your choices in different lighting as well. Don't expect to pull your fabric and start cutting right away - look at them a long time. I have yet to use the first audition I've made for a bargello.
2) Cutting and sorting - once you've chosen your fabric put post it's on them with the letter or number of the fabric. Use an end piece of your first strip cut - pin it to your design board with the number. keep it there so you'll always know that the purple pansy is #1 and the grapes are #2.
3) Sewing the strips. Make sure that your seam allowance is consistent from one strip to the next. Sew one strip one direction and the next in the opposite direction to avoid bows. I usually leave the leading tail of thread attached so I know which end I started sewing that strip. Iron every seam after you sew it, don't wait to iron them all. After you sew each strip and iron it - MEASURE IT!! This is true for both the color strips (the first step) and the bargello strips (the second step).
4) Assembly - hang the bargello strips on your design wall as shown in the directions - don't sew them together until you've hung all the strips. I usually mark them as well (1, 2, 3). When all are hung, step back and look at it - it's VERY easy to hang one strip up-side-down. Take a picture and then look at the photograph - you won't believe how many times we have not noticed mistakes until we've posted them here on the board and looked at our photo's!
have fun!!
Added one more thing ... I do this with most patterns, and especially with bargello. Make a copy of the pattern and use a pencil to check off strips you've cut (it's easy to lose count) and strips you've sewn and hung on the wall.