To dress a diamond wheel you use a dressing stick which has the appearance of a long half inch square slip stone this usually came with the diamond wheel this was a Norton wheel, switch on the grinder then switch off, then stop the wheel with the dressing stick, repeat until the wheel has been cleaned, this process clears out the ground carbide from the wheel face leaving the diamond exposed ready to use. This process only clears out the carbide and does not reduce the wheel size.
To true the wheel you use a truing tool about 8mm diameter sintered metal containing diamond particals, this is held in a vice or chuck etc and touched onto the wheel then passed across the face of the wheel this then trues the wheel by removing the diamond high spots, after doing this its best to keep wheel mounted on its arbour and remove it by removing the wheel and arbour complete, this requires arbours for each wheel if you can do this.
look at the Norton website if they are still going that is
Cheers Colin