CubeSats are an established trend in the space industry. The CubeSat standard opens opportunities for rapid and low-cost access to space. The use of COTS components instead of space-hardened hardware greatly reduces the cost of CubeSat-based missions and provides the additional benefit of increasing software functionalities at a low power consumption. However, COTS components are not designed for the space environment, making CubeSats sensitive to space radiation. This means that CubeSats need additional software mechanisms to guarantee resilient behavior in the presence of space radiation. Our proposal is that such software implemented fault tolerance mechanisms must be tailored to the specific code running in each CubeSat and the logical way to achieve that is to extend the software development process for CubeSats to include the systematic resilience evaluation of software as part of the CubeSats software lifecycle process. This paper proposes a set of structured steps to enhance the classic software development process used in CubeSats, focusing particularly on the Verification and Validation (V&V) phase. The approach uses fault injection as an integral part of the development environment for CubeSats software and includes three major steps: a) sensitivity evaluation (verification) of software in the presence of faults caused by space radiation, b) strengthen of the software with targeted software implemented fault tolerance (SWIFT) mechanisms and c) validation of the effectiveness of the SWIFT mechanisms to confirm that the software is immune to space radiation faults. These added steps to the V&V process must be carried out during software development, as well as every time the CubeSat software has an update, or even a minor change, to ensure that the impact of faults caused by space radiation is tolerated by the CubeSat software. The paper demonstrates the proposed approach using three different embedded software running in the EDC (Environment Data Collection) CubeSat board, which is part (payload) of a constellation of satellites being developed by the Brazilian National Institute for Space Research (INPE). EDC use case provides a realistic insight on the effectiveness of the proposed steps. Our results show that the proposed approach can reduce the percentage of silent data corruption (the most problematic failure mode) from the range of 15% to less than 1% and even to 0% in some embedded software, meaning that the CubeSat software becomes immune to space radiation.