How can I ensure that my code, program, or software is functional and secure?
Tetsing is the easy answer to quality. But ego and pride needs to be put aside when testing. Trying to prove or disprove that the code does what it intends and does nothing not intended is the goal. This is one of the reasons independent testing teams are popular. The other important aspect of testing is to do it. Build the tests into the CI piplines so they run as part of the process and don't leave the build broken. If the tests fail check them, fix them, do not just flip the tests off.
How do I interpret user needs and incorporate them into a program?
User needs are difficult to understand often as the user may not actually know what they want or cannot communicate it effectively. During ellicitation using the 5 Whys technique is often useful. Another useful feature is shoulder surfing - watching a user use other software and ask what they like and why they are doing certain things. Lastly, creating mock-ups for the end user can help to prevent a major mistake in the delivered software.
How do I approach designing software?
I approach the person presenting the software with with a naive curiosity. This is important so that they can help others understand their vision. Often the client doesn't know everything up front and elicitation can push them to explore their vision further. For me elicitation is the most imporatnt step in Requirements collection.