Tennis bowl does not seam or swing because tennis ball is not made for cricket. A leather ball which is made for cricket has seam (The sewing on it) which helps a leather ball to seam or swing. But you can still bowl well and deceive batsman with various art of bowling which applies to both tennis and leather ball. Here are some tips which can be helpful while bowling with a tennis ball:-
1. During your run up, grip the ball like an off spinner. Stretch your middle and index finger and hold the ball in between. Run towards the batsman and jump a little bit when you are about to reach the crease. As you land on your right feet (considering you are a right arm bowler) and follow that up with left feet, make sure when your right feet lands, you are in initial stage of your bowling action. Follow the rhythm of body from right to left leg and just when you are about to release the ball, rotate your fingers in a clockwise direction. The ball will start revolving in a clockwise direction in the air and as a result it will turn into the right handed batsman and away from a left handed batsman. Remember, the tennis ball bounces way more than leather ball and also it turns a lot on any surface.
2. The other technique you can use is to bowl an off cutter with wrist just like James Faulkner. The ball will behave similarly as mentioned in first point but the grip is a bit different. Here, instead of holding the ball between two fingers, you have to hold the ball in your palm (Similar to when you catch a ball one handed). Grab the ball tightly and go on your run up. The action should be same as mentioned in point one. The only difference is that you have to revolve the ball using your wrist. While releasing the ball, you have to rotate the wrist clockwise. Make sure you don't bend your knee in the process. You can use a combination of finger and wrist together and if you are able to control it, you will be a deadly bowler.
3. After off cutter, let's talk about leg cutter. Leg cutters are bowled with a similar grip as a leg spinner. So let's first see how to bowl a leg spin as you have asked in the end of your question about leg spin. Remember, leg spin is a tough art. No one can claim to have mastered it not even Shane Warne. Since leg spin is mainly bowled with the help of wrists, you need a lot of control from your wrists. The other thing apart from control, you will need is a flexible wrist. The more flexibility is there in your wrist, the more revolutions you can provide to the ball which will result in more turn. Bowling leg spin with a tennis bowl is a tougher job than bowling with a leather ball. Reason is that leather ball has seam which helps the bowler to grip it better while tennis ball is flat hence it's a bit tough to control it during release. To bowl leg spin, first of all you have to mark your run up. Take off from your mark and jump slightly (If you prefer because it might lead to an off length bowl but will generate more turn). Make sure your right foot lands first pointing towards short mid off (Parallel to the crease) and the left leg lands on about at an angle of 125 degree from where your right foot landed. Also make sure that your toe lands facing towards fine leg (Although it's not recommended but this will generate more turn). After landing, rotate the left toe on ground and transfer your entire body weight on left foot. Rotate entire body just when you are about to release the ball, rotate your fingers along with wrists in an anti clockwise direction. Make sure your head is still and eyes are level in the process. Leg cutter is bowled in the same way as leg spin but the difference is that you have to run a little further and act as if your are a fast bowler.
4. The last trick you can use is to bowl a fast top spin. Top spin usually is bowled by leg spinners, is a ball where the bowler releases the ball from back of the hand and instead of trying to revolve it anticlockwise, the bowler revolve it towards the batsman which results in excessive bounce. This could be a great weapon with tennis ball.
So these are the tricks which you can use to trick a batsman with tennis ball. In the case you don't know what run up and mark is, here is the definition. Run up is the part from where you start running towards the batsman. The area from where you start and up to the crease is your run up. Bowlers mark the run up by jumping in the direction of run up and then walking up to the distance of their required run up. Marking a run up reduces the probability of bowling a no ball.
In the case you want to learn more about fast bowling, you can read my article 'How to become a genuine fast bowler' in article section.