linkedin with twitter is the easiest as far as synchronisations of social services are concerned.
After logging into linkedin,there is an status update box as above;just left to the share button,the famous twitter icon is present.Click on it.You will be taken into the twitter authorisation page(thanks to Oauth,you need not to give your username/password combination to linkedin).Log into your twitter account.
After logging in,you will see a page that will ask you to allow linkedin application.Press the "allow" button.You'll be redirected back to a page as below
In this page,you have to decide whether to allow all twitter updates to be visible to your network.If you are an avid twitter user and you tweet about anything and everything under the sun,then its not wise to select "all tweets"(your network member may get irritated with those huge amount of twitter updates and unfollow you-an undesired side-effect).In this condition,its better to choose the second option-"only tweets that contain #in".If you choose this,you must type "#in" in the tweets you want to share with your linkedin network members.Just type "#in" in tweets and it'll be shared in linkedin.
Now press save settings button.
You'll be taken back to main page again.Post an update.If you want to share that update with your twitter followers also,just select the twitter icon and then share it.