1.The folder where the mapping image created with Metasequoia 4 is saved

When created from a blank image file in [Material Property], the save destination of the file is the folder specified in [Configration].

You want to go to the folder that texture files are saved. Copy the address of [Texture] input field and enter to address bar in Explorer.

Image files placed in the same folder that MQOZ/MQO file is saved are preferentially read by Metasequoia.

2.Export of UV development

Then, export the UV development. Select [UV Edit > Export] in menu.

Select [File] and specify a file name. You can save the UV development as an image file.

If you use graphics editor that has layers function such as Photoshop, you can display the UV development and mapping images on top of each other.
*It can be overlapped and displayed when blending Mode of each layer is set to "Multiply".

3. Edit with other graphic editing software

There is no need to exit Metasequoia when editing mapping images with external software. If the mapping image is changed and saved, Metasequoia will detect the change when the window is activated.

Draw a rough sketch of the mapping image in Metasequoia
Edit and save the file with other graphics editing software
When returning to Metasequoia, changes in the mapping image are immediately reflected.

Save the image frequently and try editing while checking the display on 3D with Metasequoia.