There are many ways to do mission planning. I am a big fan of visual and tactile activities for mission planning. We use an entire meeting for mission planning.
To understand the different missions and scoring, you need the challenge document. It can be found at http://www.firstlegoleague.org/challenge. You can print a copy or access it on your device.
To help the team visualize the missions, have a mission table set up.
Dot stickers make it very easy to see the points of each mission. For each mission, write the point value(s) on a dot sticker and stick it on the mat next to the mission. Stick the label in an area likely not be used. We used stickers every year with no problem, but use stickers at your own risk as they may leave a residue on the mat.
Post-it notes or index cards are used to identify attachment options for a mission or for each team member to rank missions. Ranking missions allows team members to prioritize what they want to work on. You can use these preferences to create sub-teams for robot runs.
Mission sorting cards are great for helping the team define runs and mission order. To create mission sorting cards, visit http://www.techbrick.com/ to get the mission images.
Break the board into logical zones. This works great with large teams that utilize sub-teams. Each sub-team works on their zone.