By that I mean designing with all graphical features having a size that's a multiple of two pixels and aligned such that all features fit entirely within a 2x2 grid. One common way to do this is to design everything in vector at the normal size with features aligned to the usual one pixel grid, scale up to 200% to add details and export the @2x asset, before scaling back down to export the @1x asset. For fine details, it may even be convenient to scale up to 400% or 800%. Another useful tip is to set your graphics program to show gridlines every 2 pixels.