When I try to make sketches with a fountain pen on printer paper, it keeps smudging, so I have to restart. What do I do to prevent smudging?
If this helps, I'm ambidextrous.
There are four things I can think of which will help you avoid smudging your inks are: how you hold your hands over your working surface, controlling the amount of ink on your paper, helping the ink dry quicker, and being present at all times in the drawing process.
Keep away from wet areas.
Always keep your hand above the surface of the paper, never resetting the edge of the palm on the paper. Only the tip of the pen is allowed to make contact with the paper. If you do need to stabilize your hand, for some fine detail work, you either stabilize your hand using the tip of your pinky balanced in an area free of ink or you use something like a painter's maulstick which gives you a stable surface that is balanced from the outer edges of your working area.
Remove any excess ink helps speed drying time.
Fountain pens can place generous amounts of ink onto the surface of your paper. This pooled ink takes a long time to dry. To help the drying process you can use something called an "ink blotter". An ink blotter is a special pad of paper used to soak up excess ink from the surface of the paper. Helps to dry the remaining ink faster.
Evaporate the moisture.
Use heat to help the ink dry quicker. A heat gun, or blow dryer, passed gently over the surface of your working surface will help remove the moisture from the ink.
Stop and think.
It's easy to get lost in a drawing and forget what areas you have recently gone over. It's those recently drawn areas which have the greatest change of being accidentally smudged. Be present in the moment, slow down your drawing and pay attention where your pen has been recently and where you want to go to next. Knowing what you have drawn and when it was drawn will help you avoid inadvertently running into still wet ink.
Printer paper may not be the ideal substrate for a fountain pen. Can you try different paper - something specifically made for pen and ink?
Another thing to try is to use a slanted surface. This is actually one of the keys to writing with quills, which deposit ink even quicker than a fountain pen. If your paper is at a slant, you end up holding the pen almost horizontally, so gravity isn't sending large amounts of ink onto your paper. You'll need to play around with the angle to find that sweet spot between "ink doesn't flow" and "ink flows too quickly" for your particular pen, ink, and paper, but 45 degrees is a good start.
And then there's the obvious thing, though we tend to forget it when drawing: work from the top down and left to right (or right to left if you're left-handed). If you need to go backwards -- upwards or away from your writing direction --, wait for your previous lines to dry, or help them along with a blotter or with sand.
A substance called gum sandarac has been used on paper so ink more readily takes to it. Also drafters powder. Check Amazon or Jon Neal Booksellers websites for these products.
Myself I’ve found different weights of the top quality Mohawk paper give good results.
As others have said treat yourself to a visit to an art and craft store to review the paper that’s easily available. Art paper tends to be separated by the type of media it most readily accept.
Jetpens.com also has an excellent series of guides and how tops on fountain pens and paper.
When I try to make sketches with a fountain pen on printer paper, it keeps smudging
One of the pleasures and annoyances of fountain pens is that your writing (or drawing) experience is quite customizable. Specifically, the particular pen, ink, and paper all matter in this subject.
Nibs come in a wide variety of sizes. An extra fine nib lets out much less ink than a stub nib, and more ink can mean more time to dry.
Different inks have different physical properties, so some dry quicker than others. They can also absorb into the paper easier, which helps prevent smudging (but may produce feathering you don't want).
And paper changes smudgeability quite a bit too; if it's glossier, the ink will stay on the surface more, and thus get smudged easier.
Experiment with different combinations of all these, and you may find something that acts more like you want.