You can also turn a taper by offsetting the tailstock. When the centerline of the tailstock is exactly in line with the centerline of the spindle and you are turning a shaft between centers it comes out parallel. Offset the tailstock by 0.10" and turn the shaft and it will have a 0.20" taper over its length.
You may need too much taper to do it this way but maybe not.
When the tailstock is offset you cannot use regular straight sided center drills to make the center holes you have to use the ones that have a concave taper. This puts a convex taper inside the center holes so the part can rotate smoothly. You might need to Google to find some pictures of what I am referring to.
Also you have to have the shaft between centers you cannot grip one end in the chuck.
Another way to do a taper, which is a bit hokey but can work, is to mount a flat tool at the correct angle for the taper and plunge straight in; this leaves a tapered portion on the shaft. Then move sideways maybe 0.1" and plunge again until your just blend perfectly with the first cut; and keep repeating this.
You need a steady hand for this
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Then polish it a bit with a file and emery paper. One handed filing is tricky but possible. Put a handle on the file to cover the nasty sharp tang just in case you slip.
Yet another way is to rig up a guide strip at the back of the cross slide; a sloping cam with a cam follower mounted to the cross slide.
Then you have to adapt a little 'winch drum' onto the cross slide handwheel so you can wrap a string around it with a heavy weight. The weight turns the winch which feeds the cross slide forward until the cam follower touches the sloping cam. Now you feed the saddle along and the weight keeps advancing the cross slide down the slope.