When you first set up WLED, there's one segment pre-defined, and it's called Segment 0. This segment includes all your lights: I have 200 in my left-hand strand and 100 in the right-hand strand so the total here is 300. If I run an animation on segment 0, it will run down my first strip and then seamlessly down the second or any additional strips. The LED numbers are continuous, so the segment layouts don't need to match the physical layout of the strips.

Select "solid" under effects and choose a color.

Click the "add segment" button and create a new segment. Starting with LED 1, take a guess at how many lights you should turn on to light only your top message. Click the "check" button and look at your lights. 

If all your lights are still on, click the "power" button to turn Segment 0 off and be sure the power button on the new segment is active. This should turn on LEDs 1-7 and only those LEDs. 

Verify that it's lighting up the area you want and adjust the numbers if it's not.

Let's save this first segment before we move on. Otherwise we may do a lot of intricate mapping work and then discover that poof ! It's gone forever.

Segment layout can be saved using a Preset. Click the + Preset button and call your preset All Segments. Use the default settings and click "save". 

Segments are preset-specific, meaning that different presets can (and often do) use completely different segment layouts. If you don't save your work in a preset, and then you select another preset that only has Segment 0 defined, all your segments will vanish. 

This can be frustrating! So save early and save often. To update the preset after you've added more segments, click the "overwrite with state" checkbox and then click Save.

Make a segment for the LEDs that light up each of your messages. 

For my project I have 2 LED strips on either side of the mirror. They aren't laid out exactly the same on both sides. To make this work out and light both sides of the message I made two segments for those quotes: the "Imagination" segment contains LEDs 127-135 on the left, and the "Imagination2" segment uses 232-237 on the right.  

Each segment can have its own animation and color choice. I used two different colors with the "solid" animation on these two segments to give a beautiful gradient effect.