Do you have some laminations available? Why would you like to build this yourself?
Anyway, you have to start with a certain estimated lamination size, see how much primary windings you need for a certain maximum induction (this sets the 'core'-losses and depends on the kind of laminations used). Then you calculate the number of secondary windings goes together with that.
Knowing the number of windings for secondary and primary, try to find a wire diameter for both that fits the available winding area. Calculate the 'copper'-loss for nominal load (on primary AND secondary). If the copper loss + the 'core' loss is too high for the estimated form factor, choose the next bigger form factor for the laminations and start over again...
For a lot of these steps, you will need some information about the laminations used, the nominal power a certain size of transformer can dissipate before starting to smell bad, ....
My advice, try to get some information from a specialized company, and maybe order the tranformer over there too.
Stefaan