How do you make a Dota2 in-game guide?

The option you're looking for is located in game menu, you should navigate to Learn - Hero Guides and click 'Create' button. It will allow you to create your own in-game guide with an itembuild, skillbuild and your commentaries. You will also be able to publish this guide so that other players can see it.

This appears to be the only method that works as for Jul 2018

I have found two ways to create a Dota2 guide.

1. Creating it yourself,
2. Creating it with a guide.

Below you will see both methods.

1.

You can click here to go to the guide creation page. The process should be easy to navigate through, but feel free to use check the pictures from the source below.

2.

To begin creating your build, you'll need to visit the Dota 2 Blog. At the top of the page there is a dropdown menu titled Heropedia, hover over it and choose Hero Builds from the list. You can also click here to be taken there directly.

You'll then be taken to a Steam Community login page. Enter your Steam account credentials and choose Allow when Dota2.com requests access to your information.

Once you're logged in, select the Create New Hero Build button.

You'll then be taken to hero selection screen. Locate the portrait of the hero you want to create a build for and click on it.

Once you've selected a hero you'll be taken to the Hero Build Editor screen. From here you can specify a title, create an item build and an ability build. At any point you save your work by clicking the Save button located at the bottom of the page.

Your saved build can be found by returning to the Builds page from the Dota 2 Blog. It will appear under the Steam Cloud Builds section. You can click on the drop down menu to the right of the build and select Edit to the return to the editor at any point to make changes, even after it has been published.

(Link to the source)

Nowadays, just open dota, go demo the hero you want to create a build for, once you're inside the demo choose any hero build and press the pen icon and change whatever you want to change then press the pen again to save it.

In patch 7.27d you have to edit them from inside the game client.

To do this, click on the "Heroes" tab, then click on the sub-tab "Guides".
From here, select a hero and press the button labled "Create", or use another guide as a base by selecting the guide and press the button labeled "Copy".

The web interface can't be used edit or create guides anymore.
However, you can still view the guides if you like.