The pen may not always be the reason a line looks jagged or inconsistent. A brush pen may catch in a paper fiber and drag or pull, a nib may skip over the rough edges of a textured surface, or the ink may bleed and form fuzzy edges, even though your hand is moving smoothly on an even plane. In fact, a lot of the time, it is the paper which contributes to the issue. Beginner calligraphy becomes more manageable when the right paper gives the right support for the writing utensil to move smoothly, the ink to lay down without spreading, and the true shape of the written lines to be seen easily.
The texture of the paper can be a factor because as the pen glides over the tiny, raised paper fibers, the thinner upstroke might waver. The writer may respond by pressing down more heavily to try to get the pen to keep moving forward, which causes the upstroke to become much thicker than intended. The absorbency of the paper can also be a problem, because the thin upstroke can soak up the ink and appear much thicker than intended, while the clean edges of a downstroke can turn fuzzy when the liquid ink is pulled outward along the fibers. With a poor quality paper like this, the purpose of your practice will be lost, because when you look at the work, you will never be able to tell with total certainty what it is that your hand is doing.
The paper that is friendlier to a beginner, therefore, is usually smooth practice paper. The surface will offer a more consistent resistance for the brush pen or the nib as you make your ovals, your loops, your exit strokes, and the rest of the line work involved. Smooth does not always mean the more expensive, nicer papers are necessarily better. Instead, ask yourself whether the paper gives light, thin upstrokes and whether a heavy downstroke will bleed out to form fuzzy edges or not. If the paper passes those two tests, it is almost always better to practice on those pages than on a high quality and prettier notebook.
Before you write an entire page of lines and letters, you might try out a corner of a paper sheet to make sure it is the right choice for your purposes. Create a slow upstroke and a heavy downstroke, make an oval loop, try a simple connecting stroke like an n or an m, and so on. Give the ink just a few seconds to dry, then take a look at these practice strokes. Is the ink bleeding or feathering into the fibers so that the edge of the stroke is fuzzy? If the paper is too rough or the pen is dragging in the fibers, the upstroke may break up and wobble as you try to make the stroke. If the downstroke is too heavy or the ink is pooling, the ink may bleed through to the other side of the paper, which is a sign that the paper is not suitable for that pen/ink combination at all.
There is no one size fits all approach to this, because different pens and nibs will be working on different surfaces and will react to different types of paper. For example, a brush pen should use smooth paper so that the flexible tip does not fray and fray as it glides over the raised paper fibers. A dip pen with metal nib will work best on paper that can take the liquid ink without causing the ink to spread, while tracing paper can be good for drawing over your practice sheet in order to learn or relearn a particular letterform, although the surface texture is very different. If you change which pen and nib you are using, try the same test, rather than relying on one paper sheet for every setup you might create.
There is also the factor of self-education to keep in mind. On a rough surface that causes the ink to bleed, a carefully executed stroke might look uneven, and a well-practiced change in pressure might fade into the feathered edges. This can mean that the writer may come to think that it was entirely her hand which was at fault for the bad quality lines. However, if the paper is more suitable for her purpose, she will get a clear picture of whether her baseline is shifting, whether her letters are changing in slant, and if the downstrokes look heavier because her ink is bleeding into the paper fibers or not.
Be sure to keep any sample paper sheets that you may have on hand if you are experimenting with different options, then write a small word on each one using the same pen and the same writing speed as possible. Label each sheet with the observations you made about them; e.g. whether the upstrokes were smooth, if there was bleeding, whether they were scratchy or not, or if the ink bled through to the back of the page. The best beginner paper is not the most attractive or best quality of paper, but the paper that allows you to see your pen strokes in an honest way, so that your next effort can be based on your own calligraphy rather than a bad paper.