Stuff you need to know:

Crank arm length, should be stamped near the pedal end of the arm on the inside.

Saddle height, measure from the center of the BB/crank arms straight up the seat tube. This should put you at around the center of the saddle from front to back.

Saddle set back. Drop a plumb line from the nose of the saddle and mark where it hits the chain stay. Measure from that mark to the center of the BB.

I use the same or similar model of saddle on all my bikes, so these will be pretty consistent. If you are changing saddle model, you may need to account for the overall length being different or your sit bones being in a slightly different spot.

Replicating the above 3 items gets you very close to the same fit when copying bike.

Now you can set up the front end.

If you are using the same bars and levers, then just measure from the nose of the saddle to the center of the bars. Adjust stem length as needed. If you're using different bars, measure to a repeatable spot like the center of the brake hood.

Measure nose to ground and then bars to ground to get the HB drop on old bike then adjust spacers as necessary.

Some adjustments slightly affect others, so be iterative until you get it dialed in just right.