In order to avoid confusion and errors, try to pratice this tutorial keeping the size shown on the pictures. Parameters may change if you create something bigger or smaller.

First, create a new document in RVB.

Draw a circle and fill it with 50% grey (Edit Menu->Fill->50% grey)

Copy that layer, move your 2nd circle to the right. merge the second layer with the first one.

Now, draw a square from the middle of each circle.

Fill up the square with 50% grey.

Copy that layer, so you should have 2 layers. Now we're working on the copy.

Get the selection (CTRL + click on PC, Apple key + click on macintosh).

Go under the channel and create a new one (Alpha 1). Fill the selection with white.

Keep the selection and apply a gaussian blur (Filters Menu->Blur->Gaussian blur). 1 to 4 pixels. Try to have a result as close as the picture.

Go back to the Layer Copy 1 and apply a lighting effect. Make sure to select the Alpha 1 channel as Texture Channel.

Parameters here are :

Light Type: Spotlight with Intensity : +35 and Focus : +69

Properties : Gloss : -62, Material : +69, Exposure : 0, Ambience : +8

Texture Channel : Alpha 1 (white is high) and height +5

Create a new layer, name it background and fill it with 50% gray. Move it below the other layers.

This is important, the effect will (for now) only be viewable with a grey background

Go back to your layer 1 copy and apply an Outer Glow (Layer Menu->Effects-> Outer Glow)

Keep the parameters you see on the picture.

Mode : overlay

Opacity : 100%

Blur : 40 pixels

Intensity : 32%

You can change the glow's color.

WARNING : this is a intermediary step just to explain the background problem. Do it ONLY at the end of the tutorial.

As I said before, the effect will only work on your grey background for now.

If you want to have another background, it's very easy. Make the Background layer invisible (uncheck the little eye at the layer's left).

Select the layer 1 and under your layer options, choose Merge Visible.

Make your background layer visible again and fill it with the color you want because now you can change your background's color anytime.

Now it's time to create the light inside the neon tube. Create a new layer, name it 'light'. Get back the layer 1 selection and contract it by 10 pixels.<

With the paint bucket fill this selection with white

Keep this selction and create a layer mask using this button and the bottom of the layer palette. This will all a mask next to your layer :

Apply a 5 pixels gaussian blur on your layer. Change your layer options to CoLOR DODGE with 20% opacity.

And you should have this result :

Use the gradient tool select 'black and white' and fill the selection from bottom to top.

Now change the layer options to Screen with 80% opacity

Duplicate this layer by dragging it to this icon at bottom of the layer palette.

You can now merge your layers, change your background and there you go :-)