Procrastination can waylay more than just putting a condom on. Waiting too long to take a condom off has its own set of problems. An erection can begin to fade immediately after ejaculation, meaning a condom that was snug at the start can go baggy right after the finish. The softer the penis gets, the more likely it is for semen to leak out of the condom.
Immediately after ejaculation, remove the penis, holding the rim of the condom to keep it secure. Then, carefully remove the condom to avoid spilling its contents. Wrap it in tissue or toilet paper, and throw it away.