In the Lightroom settings when exporting the files.
Use the images straight from the camera. I don't think DPP will process images exported from another program. If you need to edit them before the stack, shoot Raw (I suspect you already are), touchup in DPP, then run the stack. I suggest Raw because DPP is very limited as a jpg editor, IMO, but a decent Raw editor. You can edit the first image, then copy those edits (recipe) and batch paste to all images. If you shoot jpg and like them as is, just run them. They should look fine.
