The first thing we do is create a text layer with our desired text. Choose a large font, and then once settled , Ctrl-Click the text layer to select the contents:
Now we will hide the text layer and create a new Layer. In this new layer we will add a stroke to our selection. We can do this by right-clicking the selection with the selection tool and choosing Stroke. We want it to be 3px wide, a green color (I used #00FF18) and Outside. Once this is done, we can deselect the image and open up the Layer Styles. We will give it an Outer Glow of around 10px (this will depend upon how big your text is - see what works for you, experiment). Click Ok to accept the Outer Glow. We will have something like this:
Next, we want to duplicate this layer (right-click and choose Duplicate Layer). We will then move the duplicated layer underneath the original layer and open up its Layer Styles. We want to give it a color overlay of black - if you have a background, this will help give the sign some depth.
Now we will Ctrl-click the text layer again to select it and then choose Select > Modify > Expand, and choose 2-3px or so (depending on your image size). Then create a new layer and we will do a stroke of 1px, Center, and White. You can then merge this layer with the original neon layer if you so wish.
To give it the extra look of a neon sign, we will remove parts of the letters. Choose the eraser tool and erase parts of the original neon layer (or both - you get two slightly different effects). Here is the finish sign:
You may also want to add a background to the sign, maybe a wall to make it look as though the sign is hanging, or even animate the sign to blink on and off.