Yellow headlights are oxidized plastic, not dirty glass: sand it off, polish, then seal it or it comes back within months.

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Tools I used

Tool	Why you need it	Link
Headlight restoration kit (with UV sealant)	Sandpaper, polish and sealer in one box	Check price →
Painter's tape	Protects the paint around the lens	Check price →
Spray bottle with water	For wet sanding	Check price →
Drill + polishing pad (optional)	Faster, more even polish	Search on Amazon →

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Steps

Wash the lens and tape off all paint and trim around it, two layers thick.
Wet sand with the coarsest grit in the kit, side to side, until the yellow is gone and the lens is evenly frosted.
Move up through each grit, switching direction each time (side to side, then up and down).
Rinse and dry. The lens should look evenly hazy, with no deep lines.
Polish by hand or with a drill pad until it's clear.
Clean with isopropyl, then apply the UV sealant in one smooth, even coat. Keep it dry for the time on the label.

The sealant is the step that counts. Kits without a UV coat look great for a few months, then yellow again.
If water beads up while you sand, the old coating is still there. Keep going at that grit.
Do one headlight start to finish, then the other, so you can compare.