Instructions
Highlight "Create a Class" from the multiplayer menu and press "A," then highlight "Primary Weapon" and press "A." In this menu, highlight whatever weapon you want to use with the red laser sight and press "A" to equip it.
Highlight any multiplayer match from the multiplayer menu and press "A" to begin. Highlight the weapon class you've just created and press "A." Throughout the match, use this weapon and this weapon only.
Kill as many enemies as possible. Take down 10 to complete the Marksman One challenge, then take down 25 total to complete Marksman Two. This kills do not have to occur in one match; they can add up over several matches.
Highlight "Create a Class" from the multiplayer menu and press "A," then highlight the weapon you just used to complete the Marksman Two challenge and press "A."
Highlight "Attachments" and press "A." The red dot laser sight will now be available for this weapon. Highlight it and press "A" to use it in a match.