Creating metallic/specular maps for Standard by hand
When using the Standard shader, it expects specular and smoothness to be a part of the same texture. Depending on what tools you use, this can be surprisingly annoying to manage. For example, by default, Photoshop saves semitransparent images with transparent pixels replaced with pure white - destroying the specular map! Anything that uses Windows's own image handling will destroy transparent images too, due to an ancient, unfixable compatibility bug.
Using SmartTexture to create combined maps
Using SmartTexture, you can create texture files in Unity that automatically combine several different textures into one.
To install SmartTexture, follow the instructions here: https://github.com/s-ilent/SmartTexture
Once installed, you can create new SmartTextures within Unity by creating a new SmartTexture asset and dragging in the individual seperated textures from your project.
Using ImageMagick to combine specular and smoothness
Here is a simple, efficient way of getting your maps combined in a few easy steps. Firstly, download ImageMagick. (The HDRI compatible version is recommended.) https://imagemagick.org/script/download.php#windows
ImageMagick is a long standing open source set of command line tools for editing images. People have done amazing things using IM and batch scripts. Our script is less amazing, but useful all the same.
Here's the script:
magick convert %1 %2 -alpha off -compose CopyOpacity -composite PNG32:%1_result.pngAlternatively, here it is pre-made into a batch file.
To use it, simply select two images of the same size, and then drag the main image into the batch file. In this case, you'd select your specular map and smoothness map, and then drag the specular into the batch come. The result is a combination of the two, with the second image added to the alpha channel - creating a specular and smoothness map ready for Unity! You can then take the resulting file and use it in Unity. Make sure you pick the right one!