Feel like a level designer: 7 level editors
A level designer is a person who creates levels. He uses the available resources like a constructor: he takes scripts, sounds, models, animations, and other game elements and assembles them into a map based on the technical requirements. A level designer must create interesting gameplay, work with game engines, have good taste, and find a compromise between artistic and technical requirements for the project.

Before you sign up for a course on level design, try to create your own level for your favorite game. This way you'll get a portfolio and understand how much you like the profession. Don't use other modifications: work only with the content that was already created by the developers. Filling your locations with custom sets of weapons, armor, and game objects are considered bad taste among developers, especially when it comes to portfolios. After all, the whole art department works on creating the visual concept, and custom modifications often break the overall style and affect the game performance.

As soon as you realize that making levels is your life's work, master prototyping on Unity or another popular engine. In the meantime, here's a list of games with built-in map editors for those interested in trying their hand at level design.
Warcraft III World Editor
A level editor for Warcraft III that has stood the test of time. It has given birth to genres such as Tower Defense and MOBA and has been a catalyst for inspiration for aspiring developers of all ages. Authors of particularly successful custom maps got jobs at Valve and Riot Games.

Standard features of the editor: working with the relief and geometry of the levels, creating triggers, and redistribution of character stats. Some skilled people have learned how to attach the first-person perspective to isometrics and create shooters. All this - in the editor for RTS!

The program got a second wind in the recent Warcraft III: Reforged, which combined the capabilities of the original with updated graphics. The only catch is that the game is not free.
Dota 2 Workshop Tools
The successor of Warcraft III World Editor: free, beautiful, and easy to learn. Created maps are placed in a separate tab of the game menu and equipped with a set of tags, therefore, more people will see them. Another difference from its predecessor - the target audience. On the platform are popular fan maps with references to the gamer culture, focused on PvP-battles.
Garry's Mod
A sandbox for natural-born game designers and level designers. Create anything you want: fighting games on giant herrings, space combat simulations, crab races, and even entire RP servers. Obsolete graphics straight from 2003 and strange facial animations brought their share of chaos in an already insane process, and the presence of familiar modders will raise the level of fun to the sky. You can't build a professional portfolio here, but you can run around the servers, get inspired, and create from the heart.
Creation Kit
The level editor for Skyrim and Fallout 4 from Bethesda. Initially, it was created for the needs of projects, but later the tool went to a broad audience. Hence the difficulty in mastering it and some overload of details. All objects are grouped by theme: one folder contains everything you need for Nord's dungeons, other - Azura symbols.

Skyrim has been popular for a decade, thanks in part to the flexible features of the editor. Sooner or later, players, bored with the vanilla version, discover the world of mods and assemble their own game. It creates not only cave levels. NPC companion with its own quest, a new economic system, or a peaceful town with characters living their own lives: all this can be done even by a novice level designer with no experience.
In 2017, the developers of the sandbox game Besiege released a major update with a map editor and multiplayer. With it, you can not only build new objects but also change physics and other game parameters. Multiplayer turns Besiege into a thousand new games. Among them racings in the Mad Max-style, reconstruction of the siege of medieval castles, and its own version of Fort Boyard. The sandbox game was released in 2015, but with the straight hands of a level designer, it surprises visually.
Heroes of Might and Magic
Despite its respectable age, this game series is still popular today. If you are a strategy fan, choose your favorite part and open the program. Recreate famous battles from the game's history, or bring your favorite setting to life by creating a level based on Game of Thrones or Harry Potter. And if you want to make it more challenging, set up a random map generation and think about what gameplay and scenario would be appropriate for it.
Divinity: Original Sin 2
The local editor is geared more toward role-playing sessions, but there's plenty of room for the level designer to run wild. In "Game Master" mode, the moderators have full control over the created campaigns. They write the scenario for the game, select maps from the available list or create custom levels, set heroes for combat and social interactions, come up with riddles and puzzles, and even manage non-playable characters. Another feature of the mode is the ability for voice interaction between players and NPCs controlled by the gamemaster.
What's next?
The next step is to learn how to work with the game engines. Make a block out to the map - an early prototype level consisting only of cubes.
Putting cubes in Unity is more boring than making maps for your favorite game. But at least you show your potential employers that you can work with engines and that you're willing to focus on the most important thing - gameplay.

Created a level you want to show off to your friends or use in your portfolio? Learned the principles of prototyping? Don't rush to pack the archive and send it to the public. Instead, add a YouTube walkthrough. Make a 5-10 minute video where you comment on the level and explain the visual and gameplay choices. In the comments to the video provide a link to the file exchange site with the level, the version of the game, and a list of modifications used.

When sending out your resume for the level-designer job - attach a link to the walkthrough, not to the level itself. Forcing eternally busy HR managers to download a few gigabytes and waste their time - bad tone.