The objective of playing base defense is to stay in your own base area, and assist the chase defense and offense when needed.
The main task of base defense is to protect your flag while it is still on the flag stand. Try to damage the enemy offense as much as possible while they are in your base. However, once they leave the base, focus instead on killing off the second offense who is waiting at the flag stand to grab the flag when your chase defense returns it. You are also responsible for your offense when they come back carrying the enemy flag. You should help keep them alive, by killing off the enemy chase, and providing them with health, force points, and weapons. You can restore their health by using force heal, which restores 50 health which each application (and only takes 25 force points) - so you can do it twice after turning on force speed. You can similarly restore force points by using team energize, which also restores 50 force points for 25 of your force points. Since team heal can only be used when on the light side and team energize can only be used when on the dark side, it's necessary to quickly switch between the two in order to fully restore your offense. A good base is capable of doing this by using a "force script", which is a config file they have bound to a particular keyboard key that toggles them between force sides. (Doing this requires a selfkill, so must be timed carefully). Finally, weapons can be given to your teammate by having it out while selfkilling - the weapon will become an item they can pick up, which will give them the weapon if they don't already have it, and an ammo clip of that type of ammo otherwise.
At the beginning of the map, it's the base's responsibility to lay trip mines on their flagstand, which will slow down the enemy offense's initial attack (either one of them will have to sacrifice themselves to blow up the mines, or else they will have to stop and shoot them, leaving them vulnerable to attack). It's also a good idea to start off on the dark side, so you can team energize the rest of your team as they will all activate force speed immediately on the map starting.
After this, it's still a good idea to keep track of the trip mines' respawn time, and grab them when they are available. Reapply them to your flag stand whenever the flag is returned (unless, of course, your offense is about to cap, in which case they do more harm than good). Detpacks are also a good weapon to use to do the same thing - enemy offense can often miss them, especially if you have a different weapon out. Keep in mind that you can selfkill to detonate them - so a good strategy can be to show a different weapon than the detpacks, which will make the enemy offense less likely to figure out that they've been planted. Another option is to selfkill immediately after dropping a detpack near the enemy offense, essentially making you a suicide bomber. Done correctly, this deals a lot of damage and will often kill them. Keep in mind that you are unlikely to respawn near to the same spot though, so make sure the other offense isn't nearby to snatch the flag back up!
Here is a demonstration of a 4v4 CTF PUG match from the perspective of a base defense player: