While RedYeti's link is useful, just to save a few clicks for others let me recap how to generate a code-signing cert and to use it for code (re-)signing:
1) Create your own code signing cert:
In Keychain Access, Keychain Access > Certificate Assistant > Create a certificate.
This launches the Certificate Assistant:
Name: Enter some arbitrary string here that you can remember. Avoid spaces otherwise you'll need to escape the cert's name when using
codesign from the command line.
Identity type: Self Signed Root
Certificate Type: Code Signing
Check the box "Let me override defaults", this is quite important
Serial number: 1 (OK as long as the cert name/serial no. combination is unique)
Validity Period: 3650 (gives you 10 years)
Email, Name, etc. fill out as you wish.
Key pair info: set to RSA, 2048 bits. Does not really matter IMHO.
From "Key usage extension" up to "Subject Alternate Name Extension": accept the defaults.
Location: login keychain.
Once it is created, set to "Always trust" in the Login keychain.
2) Re-signing an app:
codesign -f -s <certname> /path/to/app
3) Verify that it worked:
codesign -dvvvv /path/to/app