Well there are a few types of Boosting, you have headshot boosting, riot shield boosting, and killstreak/nuke boosting.
Headshot boosting is when you go into a 3rd person cage match and just get headshots on each other by ones, twos or however many until the next persons turn.
Riot Shield boosting is when you get into either a cage match or a search and destroy with all your friends that want to boost (best method) and get a riot shield and one many army and the person that shoots a RPD clip into the riot shield will get the person with the riot shield points.
Killstreak/Nuke Boosting is when you go into a public live game and you have a friend who lets you kill them by tactical insertion over and over again until you get what you want but look out for people trying to ruin it by finding you lol.
Hope this helped

+rep if it did plz