Step 1: Base

Create a 7x23 base for the display with any block of your preference. For this example I will be using black wool since it contrasts the light of the letters on the display.

Step 2: Letters

Place the sticky pistons facing forward forming the letters of the sign/display. Make sure to leave one block of margin all around.

Step 3: Placing Redstone Repeaters

Now on the back side of the display:

Place Redstone Repeaters directly to the pistons on the base as shown in (1).
Create a second level of blocks above and place the next set of Redstone Repeaters as shown in (2).
Create a third level of blocks above the second and place the last set of Redstone Repeaters as shown in (3).
Finally add another set of blocks to extend the second and third level as shown in (4).

Step 4: Stairs for Redstone

Create stairs to connect in the next step the bottom and higher Redstone Repeaters by placing any block of your preference as shown above for each of the letters.

Step 5: Connect Everything

Place Redstone as shown above in order to connect the bottom and top Redstone Repeaters for each letter independently. You can place a torch or lever to make sure all pistons are activated and delete it afterwards.

Step 6: The "PWM Generator"

Place some extra blocks to make a base for what I like to call the "PWM Generator" (Works similarly as a real one).

Place a lever, a Redstone Comparator, a couple of Redstone Repeaters and connect it with Redstone just as shown above (1). Direction of placement of these matter.
Activate the Redstone Comparator and set the Redstone Repeaters in the fourth position just as shown in (2).

Note: You may reduce the frequency time that the letters take to turn on/off by getting rid of some Redstone Repeaters. The configuration above displays each letter in a timely manner. The rule I always follow is having a Redstone Repeater per letter in the display.

Step 7: The Last Redstone Repeaters

Place a couple of Redstone Repeaters as shown in (1). These connect the letters together, in other words there should be a Redstone Repeater in between each of the letters. Add some Redstone if needed to wire everything up.
Set the Redstone Repeaters to the third position as shown in (2).
Finally place the last Redstone Repeater connecting the "PWM Generator" to the first letter and leave it as is.

Step 8: Final Touches

Place Redstone Lamps on the Sticky Pistons forming the letters.
Create a margin around the letters with any block of your preference as shown in (1).
Finally, place Redstone Torches in the same configuration as shown in (2) or any other, as long as the torches activate all the Redstone Lamps when pushed out by the pistons.