1. Take a blank, properly aligned grid cell and exactly fill it with landfill. I use the ChunkyChunks mod for custom gridlines to make this easy.
4. Finally, under "Filters", check the "Tiles" option, then check "Snap to grid", then uncheck "Tiles" (unless you want the landfill in the blueprint). It's important to do it in this order. We want the landfill on when we enable "Snap to grid" so that it will use it for grid size + alignment. You should have a perfectly grid aligned blueprint. Easy!