Build your repertoire with Lichess!

Lichess loooooves opening studies. And while there are dozens of excellent studies out there, none of them are popular. Why? Probably because they don't shout their subject at you in capital emojis to distract from their superficiality.

But what to do if you want a decent opening repertoire without spending a fortune on books or courses? Of course, Lichess holds the answer. With the excellent opening explorer, you can build your own repertoire all by yourself.

If you actually can get your hands on free extra material like videos or articles, that's great! Watch or read them carefully and don't focus too much on the variations. Try to grasp the general ideas and plans of the opening.
If not - no need to worry, it will also work without outside help.

Oh, and one more thing: always use the Master database, not the Lichess database. In the opening explorer, click on the little cogwheel and select Masters. Done!

Step 1: Build a variation tree

The first chapter of your study should be an overview of all variations, to which you can always go back to for quick advice. To build such a variation tree, begin with the starting position of your opening and click through the most common variation up to move 9-13, depending on the opening. Stop once you see that the amount of games is split roughly evenly over several different possible moves.
This is the main line of your opening which you will probably encounter most often - it is therefore the most important line in your repertoire. Now go back move-by-move and each time save the 2-3 most common alternatives of your opponent as sidelines. You can also play through these sidelines up to around move 10. Don't go in too deeply! It's supposed to be an overview, not a jungle.

Step 2: Structure your study in sections

It is most convenient to structure your study. This way, you can first study the main line and then work your way backwards through the variation tree, such that later sections then deal with your opponent's alternatives. For example:

Section 1: Main Line
Section 2: Other 6th moves
Section 3: Other 5th moves

and so on. Usually, every section contains about 3-5 chapters.

Step 3: Build the individual chapters

The individual chapters should handle one variation each. A good rule of thumb is:

If the variation has a name, it gets its' own chapter!

Chapters should also concern themselves with sidelines and important alternatives. You can go into much more details here than in the variation overview. You don't always have to go for the most-played move yourself: if there are alternative moves for you which score really well, they are a good candidate to look at.
Model games are super useful for studying openings. While browsing through the variations, keep a lookout for the Top games in the opening explorer. Maybe there was a top-level game won in this variation just recently? You can view the game, cite it or insert it to make it the model game for this variation.

Hint: If you can find a top player that uses this opening regularly, that's a gold mine. Study his/her games and shamelessly copy their ideas!

Step 4: Perfect the chapters and understand the opening

Once you have found model games, it is time to comment on them yourself. Make extensive use of the glyphs and arrows and use your own words to explain moves or set-ups. What are the plans, what do you have to look out for?
You have hit the jackpot if you can make out rules or patterns for yourself, for example:

Exchange knights when White threatens to retreat his bishop!

This shows that you understood the opening really well and also helps you memorizing - it is much easier to go by rules than pure memory of variations!

Edit, 10.09.2021:
@heroku made me aware of his excellent bot @openingsexercise which assists you with learning your newly-built repertoire by heart. Try it out!