you could do it in different ways...
the easiest - with shader only - is the "add" function,
but this is usually used for glow stages:
blendfunc GL_ONE GL_ONE
The other possibility is to use alpha functions.
The texture you want to blend must be saved as PNG or TGA with an Alpha channel,
which tells regions of the image that are more ore less transparent (depending on the gray value: black: totally opaque, 50% grey: 50% transparent, white: totally transparent - or the inverse).
The transparency degree can be also adjusted by the shader.
Using alpha channels, there are much more possibilities,
but it is also more complex.
You would have to play around with the shader editor.