UV Mapping with Mirror Modifier in Blender 2.9

I created a T-shirt model for my 3D characters, but this design is too simple and not interesting for me.
It need to be more fashionable.

For that, I had to UV Mapping and add a fashionable texture to this simple T-shirt.
However, this mesh has the Mirror Modifier. I struggled a little bit.

This article explains how to UV mapping for the Mirror modifired Mesh.

By the way, I assume that you already created your own mesh with the Mirror Modifier.

1. Edit the Mirror Modifier Property

First of all, select the mesh that you created and click the Modifier Properties. Then check the Mirror U.

2. UV Layout

Next, switch workspace to the UV Editing.
Make sure the workspace on the right is Edit Mode, then press A key to select all.

Then, click the UV menu and Unwrap or Smart UV Project.

You should see that the UV layout changed on the workdspace on the left.

To make it easy to put a texture later, edit the position and rotation, and so on.

First, in the workspace on the left, press A to select all.

Rotate 90°.

Use "Move" and "Scale" to place it so that it fits in the right half of the box, as shown in the below image:

At this point, this layout shows only one side, so we must be able to see the other side.
To do that, click the down arrow icon (named Overlays) and check "Modified Edges".
As a consequence, the other side of layout displayed!

This make it to much easier to put a texture. However, the both layouts are a little apart, so let's edit.

Move all the X coordinate of the points near the center line to 0.5.

As a result, it becomes look like as follow:

In this tutorial, we're going to put an illustration on the front of the T-shirt, so this layout is enough.
If you are not satisfied, you can change the layout by adding a seam. But this article does not explain it.

OK, after that, export the layout.
Click the Export UV Layout on the UV menu.

3. Create a Texture

Open your favorite Image Editing Tool like Adobe Photoshop, Affinity Photo.

Then open the layout image which you export just now.

Please note that this exported layout image shows only one side.
To make it easier to work, duplicate and invert it as follow:

Fill the entire area with the base color of the T-shirt.

Then, place your favorite illustration around the center.

Finally, hide the UV layout layer and export it as .png or .jpg.

4. Create a Material

Go back to the Blender, switch the workspace from the UV Editing to the Layout.

On the Object Mode, select the T-shirt mesh and click the Material Properties. Then create a new material.

Click the yellow circle next to Base Color and select Image Texture.

After that, click the Open button to choose the texture that you created in section 3.

Switching to the Viewport Shading, you should notice that the T-shirt became fashionable!
...To be honest, it's not a very good sense of color, it's not important in this article. Don't warry.

5. Fix distortion

As you probably noticed, the above design is slightly distorted.
To fix, switch to the UV Editing and edit the position of points in the UV layout while checking on the screen on the right.

Ok, completed.