Print the parts, i have added multiple parts in each stl file so it should be easy to print.
Use a different color for the digits then the rest. I printed my parts with 0.15 mm layer height.

Extra parts requred:
Superglue and one rubber band

First make sure the digits can slide freely in their slots on the bottom part, some filing might be required, but do not file to much so they can twist in the slots.

When you can move them with minimal friction, then put the corresponding knobs on top, make sure they still can slide but have no play in other directions. Apply some superglue to hold the knobs and digits together.

The center digit is different, it should be placed in the bottoms center slot with its small top extrusion facing the circle that have the shortest part of the slot. The center digit knob should be placed on the digit extrusion also facing the center of that circle. When done and it moves without friction, then apply some glue.

Now take the two bottom index wheels, the ones with a box sticking up of them and place on the bottom part. The smaller index wheel with the pointy 7 should be on the circle with the shortest center slot as mention previously.

Now put both the index wheels with their number 7 towards the center digit knob.
And keep them in this position when assembling the other index gears, first it might be a good thing to temporary mount the two spur gears, and turn to see it the center digit moves freely, if not some filing might be required.

Now its time to mount the other index wheels, every wheel is numbered. The numbers represent the current digit being displayed, but also one of the numbers on each index wheel have a arrow on top of one of its number, that number represent what segment it belongs to.

The segments are numbered from top, left to right, the top segment are 1, the top-left segment are 2, the top-right segment are 3, the center segment are 4, the bottom-left are 5, the bottom-right are 6, and the bottom segment are 7.

So now its time to add the indexing wheels and match them upp with the segments. And if you have the bottom indexing wheels pointing with 7 to the center, then when you mount lets say number 2 indexing wheel, its number 7 should also point at its segment.

When all the indexing wheels are mounted, then mount the spur gears on top an put a rubber band around all the digit knobs.

Now make sure you can turn it and that the display works, here some more filing or sanding might be required, and also some universal oil or lubricant might be helpful.

Now put the assembly in the front part, and make sure it still works. If everything works, now put some glue to mount the front part, be careful not to glue any segments.

Final step is to mount the two bottom knobs to the bottom with some glue to secure the index wheels and gears from coming loose.

Good Luck, have fun !