The main article for this topic can be found at Level Building.
The Level Editor was first released in winter 2000, as an addition to Tomb Raider Chronicles, the fifth game of the series. Since then the level builder community has grown rapidly. New tools were created that for example allow usage of custom objects and high resolution textures. The biggest advancement are the Next Generation Level Editor, trep and the Tomb Editor.
Since the release of the Level Editor over 2000 Custom Levels have appeared for download on the internet. Most of them are listed on TRLE sites like trle.net or Laras Levelbase where they can be downloaded and reviewed. You can find everything from badly done single-room levels to huge, impressive level series that introduce new gameplay elements and perfect textures and lighting, some of them can compete with the original games. Gamers who can not be bothered with the installation routine for the Level Editor will now find help in two independent tools, the Level Manager and the Level Player. Some of the newer and bigger games even make an installation unnecessary, as all one needs to play the level comes with the download.
For new level designers it is recommended to rebuild the tutorial level described in the Level Editor manual. Only a few level builders were able to learn what they did without the manual. It is not received well by the level building community when a new designer asks questions that are answered in the manual in a forum. Besides the official manual there are many tutorials written by fellow level builders on the internet.
For the start it's important to learn the basic functions of the Room Editor, with prj project files, with wads and familiarize oneself with the basic terms like square, click, trigger, OCB etc.
Various websites about the Level Editor offer level building tutorials.