How do I get glass walls without borders?

When I place glass walls, it shows the outline of every glass block. I see you-tubers like DanTDM or PopularMMOs have glass walls that look like one big glass block, with no block outlines. How do I get the glass without outlines?

I assume you mean something like this:

AFAIK, you can only do this with a mod called Optifine:

1. Close Minecraft and the Launcher.
2. Go to the download page and download Optifine HD Ultra for the MC version you are using.
3. Open the .jar file you just downloaded (using java!) and click install.

When it is finished, close that and open Minecraft. There should be a profile called "Optifine". Select it and click play. All of your glass blocks will be changed to look like in the first screenshot.

Aside from texture pack possibilities, there are two different types of glass in the stock game -- Glass, and Glass Pane. One looks like a window with an outline, the other looks more like a wall of glass.

You can create 16x Glass Pane using 6x Glass and a Crafting Table.