Other people have provided step by step procedures for this, but I have noticed that a few are out of date, and are hard to follow. This is my guide to signing your own application (using rotateme as an example) - This is subject to change:
1) Create a Folder in your Main Drive for the application that needs signing (eg: c:/rotateme)
2) Save the file you want to sign (in this case, "rotateme_unsigned.sis") INTO the folder you made in step 1.
Download these files and do the same:
http://www.nokia-n95.net/download/applications/signing-files/download.html 3) Go to Symbian Signed's Website
https://www.symbiansigned.com/app/page and register a new account (
PLEASE NOTE: Symbian Signed does not accept public account emails for registration, this means no hotmail, yahoo, gmail etc. You will have to find your own non-public email to register with. I am sorry I cannot help you with this) - I used an email created on a friends private server.
4) Once logged into Symbian Signed, click the "My Symbian Signed" tab
5) On the left hand side of the page, click "Open Signed"
6) Download the "Devcertrequest" at the bottom of the page shown and put the file into your folder made in step 1 (eg: c:/rotateme)
7) Install the Devcert.exe file and then run the program
8 ) Specify the output file as rotateme.csr then click NEXT
9) Key File Information: for "Publisher ID Available", click NO. For the "Private Key file", type in for our example; C:\rotateme\rotateme.key and for "Private key file password", type "12345" then click NEXT
10) Fill in all your information and click NEXT
11) Key in your IMEI number (you can get this by pressing *#06# on your phone) and then click ADD, then NEXT, then FINISH
12) When finished, you should see the following window saying that the file has been created.
13) Now that that has been done, go back to Symbian Signed and log back in. Click My Symbian again, and this time click Open Signed, then REQUEST
13) Type the letters in the security field, then click BROWSE and upload the .CSR file we have just made in step 12 (if done correctly should be in C:/rotateme etc) then click SEND
14) Now back to the left hand side of the Symbian Signed website; underneath Open Signed, click MY DEVCERTS
15) You will see the devcert you have just made. Click DOWNLOAD next to it and download it directly to our folder (eg: c:/rotateme)
16) In step 2, ensure the downloaded files "signsis.exe" and "auto_signsis.bat" are saved to your folder (eg: c:/rotateme)
17) Right Click "auto_signsis.bat" and click EDIT, which should open up notepad allowing you to edit the file
18) change the writing in notepad to the following and save:
[signsis rotateme_unsigned.sis rotateme_signed.sis rotateme.cer rotateme.key 12345]
19) Run the "auto_signsis.bat" and if all is done correctly, a new SIS application file should appear in the same folder. This is your new application installation file which should now install correctly on your phone.

I hope this walkthrough has helped most of you
If there are any questions, Please post them here I will do my best to answer them! Good Luck!