This is actually the main reason share2steem was created. I got the below tips from share2steem official Steemit page and I am sure it will be of great help for you.
1) You have to visit www.share2steem.io and click the blue button "Steemconnect Login". You'll be taken to a Steemconnect page asking you an active permission. This is necessary for Share2Steem to be able to post your tweets, Instagram and Youtube posts onto your Steemit account, and vote for you if you activate the curation trail.
Eenter your Steemit username and your active key, and validate. Your Steemit account is now registered and you're taken back to Share2Steem.
Next, you will have to link your social accounts to Share2Steem. To do this, select the social media on the left menu. Then, in the main window, add your username (the one used on the social media, often beginning with @, please don't enter the @), and you can customize the title in the field below. Instagram accounts can not be private !
2) This title will be used on all the Steemit posts relating to this social media. Then click save updates.
You can now see a necessary validation operation, which is mandatory to prove the account belongs to you. There's a little string of 8 character showing. All you have to do is copy/paste this string and the #share2steem hashtag in your next social post (for the social media you're linking).
Let's assume you want to link an Instagram account :
Once you have your verification code, open Instagram, choose your photo, and in the description field, enter your description, but don't forget to add the verification code and the #share2steem hashtag.
visit https://steemit.com/share2steem/@share2steem/share2steem-how-to-use for more info.