Hi guys. I am absolutely obsessed with this stuff so please forgive my long windedness. It is, in my opinion, really worth taking gentle steps towards a doosra just to see if you can bowl it after some work. I think that a lot of offies might be able to get there if they actually worked at it but are worried about injury or chucking. You might not be able to get there but it will increase the flexibility of your wrist just trying and that helps your ability to whip through your stock ball a lot.
All it needs to really be is a scrambled seam top spinner bowled from around the wicket (which is how Ajmal does it, albeit with a slight bit of side spin away from the right hander). The problem with the carrom ball a la Mendis, the knuckle ball a la Narine and the conventional away swinging arm ball is that they all skid a bit. This make them perfect for lbw and if you spin your stock off break hard enough and with a good amount of top spin then the contrast between the bouncing turning offie and the skiddy other one is useful for unsettling a batsman's cut and pull shots. However, if you really want to cause problems you have to be able to beat the outside edge with a variation and for that you will need the topspin that gives dip to draw the batsman forward.
Always bowl this cross seam. Your first finger should be able to pull on the seam at the top of your action and if you paused time and looked up you should be able to read the makers name on the ball you are using.
Get an incrediball and practice slowly from less than 22 yards. The first finger should be pulling over the top of the ball. Far more important is what you do with your second finger and thumb. Ajmal mimes pulling a trigger when explaining the doosra. You want to be pulling the second finger towards your palm as if firing a gun at the point fielder. The thumb is the other crucial factor. You can sometimes pick Ajmal on the tv by watching his thumb and it was famously one of the things to watch when Saqlain bowled. Basically you want to stick your thumb away from the ball and try to be conscious of bringing it directly over the top. It should pass over the ball as high as you can get it.
Also really important is to get as chest on as possible as early as you can in your action.
As for the chuck.......
A LOT of people don't understand this.
The doosra has to be bowled with a slightly bent arm. This is to get the wrist right round to where it needs to be. It is NOT for imparting revs on the ball. It is in fact really hard to bowl a proper doosra if you straighten the arm too much because it pulls your wrist back around to the right. There is, however a need to chuck for the average bowler. I say "need" because it feels that way. Just like it feels necessary to use a lot of bottom hand when hitting over the top and feels necessary to jump a bit when trying to avoid a bouncer. These feelings are the reason good technique is hard to come by. We can practice getting around them.
When you start with a real ball you will immediately notice the main problem with bowling the doosra. Once you have arrived at the crease and landed from your jump you will be very chest on. You're arm will come around with the back of your hand facing the batsman and the gun fingers towards point stuff I mentioned ready to go. The big big problem is that all of the energy into the ball is taken away. It just doesn't feel like you could physically propel the bloody thing down there, regardless of how it is spinning. Your arm is slightly bent to achieve the right wrist position and here is where the chuck happens. This is incidentally the reason I do not bowl off spin. I love mystery spin in all it's forms and found (like countless others before me) that the leg break/googly/flipper/pie set up is easier.
BUT it is totally possible to deal with this problem. It takes strength or an action change. Each doosra bowler deals with this differently (apart from those that just chuck- like Shillingford might unfortunately be doing

). Murali got the propulsion from his wrist strength and flexibility. But once again Ajmal is our best current example. He uses that pause in his action to brace and lean back before pushing hard off that base to deliver the doosra. He is a surprisingly stocky and strong.
In conclusion I have worked on the doosra with friends and teammates before and it always feels like they could get there with more practice once they figure it out. Look at George Dobell's recent comments on mystery spin. It just has to start being more accepted. Remember, you just want get to the point where you can bowl a pure top spinner and the odd one will have that 'towards first slip' doosra spin on it. It will also give your stock delivery a lot more top spin and in my experience it is is the number one problem with off spinners that they spin the ball too squarely and get less purchase than they should.
Cheers.