Create Certificate Signing Request
Start by creating a .certSigningRequest (CSR) file on your Mac, using Keychain Access. Open the Launchpad and type "Keychain" in the search.
Next, open Keychain Access > Certificate Assistant > Request a Certificate From a Certificate Authority.
Enter the email address that you use in your Apple developer account, and enter a common name. The common name can be anything you want, for example a helpful descriptive name like "ios-mybiz". Check Saved to disk and Let me specify key pair information, then click Continue.
Give your CSR a helpful descriptive name, such as iosapp.certSigningRequest, and choose the location to save it on your hard drive, then click Save.
In the next window, set the Key Size value to 2048 bits and Algorithm to RSA, and click Continue. This will create and save your certSigningRequest file (CSR) to your hard drive.
In the next screen your certificate creation is verified. Click a button to view it, or click Done to go to the next step.
You also get a corresponding public and private key pair, which you can see in the Login > Keys section of Keychain.
Double-click on your new private key to open the Access Control dialog. Check Allow all applications to access this item.
Now login to the Member Center on https://developer.apple.com/. Click Accounts then you have to click on Certificates.
Next, click the add button (the little plus sign) or Create a certificate
Next choose the iOS Distribution (App Store and Ad Hoc)
Click Choose File button and select your previously created CSR, in this example your iosapp.certSigningRequest. Press Continue
Your new certificate has been created. Press Download to download it to your Mac.
Your certificate can be found in Downloads under the name of ios_distribution.cer
Remember to make backups of your keys and certificates and keep them in a safe place.