If you just want to actually make your own CI surface plate flat, then you need another KNOWN flat one that is BIGGER, not smaller. You may be able to norrow one form another forum member for example.
You blue the bigger one and srape the high spots of yours until you have an even distribution of say 20 or 40 spots per square inch, depending on the purpose. If yours was ground first (not scraped), then you have to 'break up the surface' by a couple of gentle shallow passes 90 deg to each other first. That way you can get meaningful 'prints'.
Once you get the hang of it, it actually doesn't take all that long. Mind you, a medium sized surface plat isn;t the place to start learning! Way too likely to frustrate you. Learn on something about 4"x 6" first.
Cheers,
Joe