How to Make a Grid

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-  Set the page dimensions in pixels/cm

-  Set the resolution to 72ppi

-  Go to preferences -> units and rulers

-  Make sure that the ruler unit is in pixels

-  Command + r to show the rulers

-  Window -> info to see the measurements as they're being drawn out

-  Drag from the horizontal/vertical ruler and along the page, stopping at points of equal distance to draw out the grid to the required size