The blender. How to combine multiple textures into one - Stack Overflow

I have a character model *.fbx, and 4 texture images in separate files. 3 1024p images and one 2048p. I easily combined them into one 3072p picture. But I absolutely can't figure out how to use this one new texture in the Blender now. When I select the entire model and click Tab to move everything to a new texture, they all stack on top of each other. How do I highlight something specific, how do I compress the grid proportionally by 2/3 and 1/3 so that it fits perfectly to the created image?