Being a good Twenty20 cricketer is exactly the same as being a good cricketer. So save yourself the trouble of working out how to adapt and focus on your short-format skill development.
Quite the claim?
Let's look at the facts.
Take out the shortened time and all the basic elements of success in Twenty20 cricket are usable in any format. However if you flip it around and try to play 5 day cricket style sped up, you fall down. So start with Twenty20.
Here are some more specifics for your skill set:
The cliché of short format batting is the slogger who creams everything over midwicket. In fact, if you look at the best Twenty20 batsmen, they all play with great skill and adaptability. Chris Gayle is the Bradman of T20, with a strike rate of over 150. He also has the ability to take time early in his innings to get set, then play shots all round the ground with tremendous power. He has converted this to Test cricket with great success.
Of course, Gayle is one example to consider, not to copy. Others with huge success do things differently. Brad Hodge and David Hussey are able to hit gaps and run hard alongside effective big hitting. Raina oozes classical class and confidence as he scores at over 140.
Twenty20 has thrown off the shackles of old school "proper" batting. Playing straight is still important, but so is adaptability and that has allowed players to develop their own techniques that would have been throw out before.
So, the adaptable skill base for batting includes:
At first glance, bowling is the most different across formats. Longer games means taking wickets whereas limited offers are about restriction of runs: opposites. But in reality, it's all about making plans work through accuracy.
Where your tactic is to "bowl dry", set batsmen up, or slow scoring rate and range through variations, the common element is to hit your target as often as possible.
If you can bowl a good length and mix in variations with accuracy (yorkers, googlies, and so on) you are a formidable bowler. The rest is simply down to the tactical approach you take. You bowling practice will change very little:
So far you may or may not have agreed that T20 is the template for all cricket skills. However everyone agrees that it is perfect for fielding.
Every run counts so much that fielders are now performing acrobatics to save a single. That's exactly the same from Test cricket down to a local Under 11 game. Twenty20 fielding standard have become simply fielding standards.
So practice all fielding like that:
For me short format cricket is the best framework to work from because it is so flexible. You still learn so the important skills of longer formats, but you also can develop individual flair because you are unrestricted by dogma.
So practice your skills like a Twenty20 specialist and you may find you accidentally become a long game specialist too.