How to design levels on the paper
Designing levels of a platform game on the paper is possible. A beginner game designer can start like this.
How to
Take a blank sheet of paper, position it horizontally, and draw two long lines across it to divide it into three strips.
Now you have created a level, a side-scrolling environment.
Transfer into digital form
The level editor of the game should be comprehensible. You need to gain the knowledge, how to transfer your creations into the digital form.
For example, the Doom editor includes terms like sectors and vectors, which makes the transfer more difficult.
How to design levels in the digital form
The level editor should should be designed as good as possible, because the user will work with it a lot.
A level editor with many tools is a good, but the user of the level editor needs to know, how to use it.