The first photo shows squaring-up the end of the tube against a disk sander.
Shaping the venturi...... try to visualise what you are trying to achieve here. To get the 'venturi effect', that is suction, you must have the narrowest part of the bore where the NVA passes through. There should be a straight taper from the intake to where the venturi narrows and another straight taper from the NVA to the crankshaft.
The second photo shows a drill-bit held in a vice and the venturi held in the chuck of a pistol drill. This removes a lot of material very quickly. Final shaping is with small round files. Again, the the venturi can be spun by the drill and the file held in a gloved hand.
A vernier calliper is worthwhile investment here, if you haven't got one, to measure the size of the bore.
Not that I want to encourage anyone too much, but, an adjustable reamer will allow you to get the right size within a few thousandths. Drill a hole for clearance, then ream it to the final diameter by adjusting the reamer, do a little, measure, adjust larger, measure, until you get just what you need. A few thousandths is a noticeable change, so you need to both be able to ream it to the right size ,and measure it to a few thousandths, to have any hope of reliable results.
Also, a split-ball type small hole gauge is necessary to get reliable measurements. Adjust the ball so it just goes through, then measure the ball with a micrometer. Both micrometers and the hole gauges are usually available for modeler-type money used on eBay. Calipers will get you pretty close but will tend to measure slightly under, because of the width the the blade at the measurement end.
Brett