Multiply every measure by 25.4. ;D
Select stock material of the closest size (1/4" ==> 6mm) and make allowance for the difference where it is easier to put it.
i.e. You can make the columns of a table engine .35mm longer to get back the missing metal on the table, then cut the piston rods to the original size.
On complicated engines, write down on the drawing itself the converted values and/or the chosen metric size. Get Yourself an imperial drill chart with metric equivalences (drill #56 ??? ) and a screw chart too.
Pay a strong attention into writing clearly on the drawing the metric thread You selected in place of the imperial one, ON ALL THE PARTS. (You guess why I tell You that).
If You have an Euro converter (they can still be found on street markets for bucks, here) You can set the 'rate' to 25.4, then key in the imperial rate and read the metric equivalent rounded to the hundreth/mm. Some will allow doing calculations, too.
An inch rule and an imperial caliper are in order.
Welcome.
Marcello