The objective of playing chase defense is to defend your flag, and chase down the enemy flag carrier and kill them to return your flag back to your base.
When the map starts, you will spawn near your base. Your first task should be to run and pick up one or two strong weapons. These should be weapons that are capable of damaging players even with their saber up, and the more damage dealt, the better - you want to return your flag as fast as possible. Chase favorites are the rocket launcher, imperial repeater (almost exclusively for its alt-fire blob, which deals splash damage), and golan (capable of dealing a lot of damage, though harder to aim than the first two). On some maps, a concussion rifle is also available, though these use up ammo quickly. Detonation packs can also come in handy when planted on your flag stand. Don't waste time at the beginning putting trip mines on your flag - this is your base defense's job.
You should position yourself towards the outer edge of your base until you see the first enemy offense coming. You want to strike them quickly, before they are deep into your base, to make your job as easy as possible. Chasers often selfkill even before the enemy offense picks up the flag, in order to respawn with more force points than the offense has, and not be outrun. The easiest time to return the flag is before the enemy offense returns to their base - keep this in mind. Once they return, they will have the advantage - you will need to spend force points using speed to reach them, and they have an entire base to run around in while they wait for their flag to return. Use force pull and push to reduce their speed and allow you to catch them before they take off, but use it judiciously - most offense will have force absorb activated, so they will gain valuable force points from it. A well-placed pull can cause even the most careful offense to slip and fall into a pit, so the best times to pull are often when players jump over gaps, or the pull would cause them to fall and take damage.
If the offense does escape to their own base, then you have your work cut out for you. You will have to out-smart or out-skill them in order to take them out with your positional disadvantage. They will be aided by their base defense, who will top off their health and force points while you respawn and run to their base. You will also have to avoid being killed by the baser while you track down their flag. Fortunately, your second offense should be able to provide some support, as long as they can still get to the enemy flag stand quickly if needed.
If you successfully pull a player into a pit, the player will often refrain from selfkilling for as long as possible, in order to warn their teammates that your flag is about to return to your base, so they can get into position to immediately re-take it. You should do the same, by sending a team-chat to notify your players. Most chasers do this by saying "cap".
Another possibility is that you kill the enemy offense while they are jumping over a pit, causing the flag to fall to the bottom of the pit and not return. This is called "flag overboard", and you should notify your teammates by saying "flag overboard" or "ob" in team-chat. Normally, a dropped flag will automatically return to its base after 20 seconds. However, it is possible to expedite the process by intentionally falling into the pit directly on top of the flag, which will cause it to immediately return (just before your untimely death).
Here is a demonstration of a 4v4 CTF PUG match from the perspective of a chase defense player: