Most of the highest-impact desk posture fixes are free adjustments to how you use existing furniture, not purchases — Mayo Clinic's office ergonomics guide covers the fundamentals.
Screen height
The top of the screen should sit roughly at eye level — a laptop propped on books, or an external monitor raised, prevents the forward head-tilt that strains the neck and upper back over a full workday.
Chair and hip position
Hips should be roughly level with or slightly above the knees, feet flat on the floor — a rolled towel or cushion can substitute for lumbar support if the chair itself doesn't provide it.
Elbow and wrist alignment
Elbows near a 90-degree angle with wrists in a neutral (not bent up or down) position reduces strain that, while more associated with the arms, often correlates with the same overall slumped posture that stresses the back.
Is a standing desk necessary to fix desk posture?
No — good seated ergonomics plus regular movement breaks address most of the same issues a standing desk targets; a standing desk can help but isn't a required fix.