There's not a single proper way to use a dumbbell while sitting down. But that's because there are a bunch of ways to properly use a dumbbell while seated — if you've got a dumbbell or two and a sturdy seat, you've got workout options.
And just because you're going light on equipment doesn't mean you're skimping out. As reported in a 2016 study from The Journal of Strength and Conditioning Research, dumbbell exercises actually engage some muscle groups more efficiently than more equipment-intensive barbell and Smith machine workouts. So let's start with three dumbbell classics.
Seated Shoulder Press
The dumbbell shoulder press — which targets the deltoids while also working the triceps, traps, biceps and pecs — is just about the most straightforward seated dumbbell exercise you can do.
Start in a sitting position with your back straight (if your seat or bench has back support, even better), butt flat on the seat and feet flat on the floor, or positioned on the bench's foot pad. Hold the weights so that your palms face out, beginning with the dumbbells at your shoulders and your arms positioned so that your elbows and wrists are in line.
Keep your core tight as you inhale, then exhale and press the dumbbells upward until your arms are extended, so that the outward-facing weights meet. With a smooth and controlled movement, lower them to the starting position to complete one rep. Throughout the exercise, be careful to avoid bending your wrists, and always keep your back straight rather than arched. 
If you want to work the lower biceps (a.k.a. the brachialis), upper biceps and traps but all you've got is one dumbbell and a solid place to sit, concentration curls are your ticket.
Seated on a bench or chair with your knees out at roughly 45-degree angles and feet flat on the ground, hold the dumbbell between your feet with your palm facing outward. This should position you so that your arm is extended and your bicep is resting against your inner thigh.
Bend your arm at the elbow to raise the weight up to the front of your shoulder, then lower the dumbbell to the starting position. Since you're working with a single weight, make sure you perform an equal amount of sets and reps with each arm.
For a biceps-blasting workout that requires just one dumbbell and a special seat called a preacher bench (standard equipment at most gyms), add dumbbell preacher curls to your upper-body day routine.
Seat yourself on a preacher bench with a dumbbell in hand, placing the back of your bicep against the bench's curling pad — your armpit should be right on the top ridge of the pad, so a little adjustment of the seat may be necessary. Brace your core to help stabilize your spine and assume the exercise's starting position with your arm extended and palm facing out, leaning your body into the pad and resting your other arm atop it.
With a slow and controlled motion, exhale as you raise the weight up to your shoulder. At the top of the movement, your forearm should be vertical and your palm should directly face you — keep your wrists in line with your forearm and your spine straight throughout. Inhale as you return to the starting position. As with any one-handed dumbbell exercise, perform an equal amount of sets and reps with each arm for a balanced workout.
Don't Stand (But Do Comment)
What's your experience with seated dumbbell workouts? If you've got a seated dumbbell exercise that you love — or any killer tips for seated shoulder presses or dumbbell curls — make a comment below. You won't even have to stand up for it.
- Breaking Muscle: The Overhead Press: The Actual Difference Between Seated, Standing, Dumbbell and Barbell
- ExRx.net: Dumbbell Shoulder Press
- ExRx.net: Dumbbell Concentration Curl
- ExRx.net: Dumbbell Preacher Curl
- The Journal of Strength and Conditioning Research: Maximal Strength Performance and Muscle Activation for the Bench Press and Triceps Extension Exercises Adopting Dumbbell, Barbell and Machine Modalities Over Multiple Sets
- ACE Fit: Shoulder Exercise: Seated Overhead Press
- ACE Fit: Arm Exercises: Dumbbell Preacher Curl