In a departure to the method described by Martin Cleeve, rather than clamping the parts, I simply super-glued then all together for drilling. Make sure all surfaces are clean and degreased. Start by glueing the end T-piece flush with the square end of the base plate, and the finshed edge.
To space the t-slots, I used an 8mm drill plus one thickness of paper. This gave 8.13mm spacing. Glue each T-slot piece in turn, feeping the machined ends flush with the finished edge of the base plate.
To glue the dovetail pieces, first place them on the saddle dovetail (don't forget the gib strip). Then place the basplate on top roughly, centered on the two dovetail pieces. There should be about 3mm showing each side. Now use your DTI to get the base plate square to the bed. Once it is squaare, carefully run superglue along the crack betwee the front dovetail piece and the base plate. Capilary action will draw it in. Check the squareness of the baseplate, and that the t-slots are parallel with the bed with the DTI. if anything is off, it is easy enough to lever off the parts with a screw driver , scrape off the old glue and try again.
Glue the Tailstock side dovetail piece to the baseplate while gently squeezing forcing the two dovetail pieces down and into the saddle.
Turn over and run some glue along the inside of the baseplate/dovetail cracks.
Blue the top and mark out and centerpunch the screw hole pattern. Drill the whole pattern (except the feed nut holes) first with a small center drill, and then to the core drill for M5 4.2mm.
Lever the T-slot pieces off, and then clearance the base plate and dovetail holes though to 5mm. Then disaasemble everything, scrape all the super-glue off, countersink the dovetail pieces lower surfaces with a 10mm 90° countersink. I had to redo this step when I changed from M5x16 to M5x20 screws, as the heads on the longer screws where also thincker. While you have the countersink in the drill press, deburr all holes except the top of the T-peices. The holes in the 0.5mm shims were deburred by filing flat. Tap the T-slot pieces to M5 from the back.
The tailstock side dovetail piece can be marked out for the gib adjusting screw holes, drill and tapped.