This paper presents comprehensive guidelines for the design of alternative energy aircraft, with a focus on battery-electric and hydrogen fuel cell powertrains. Traditional first-order models like the Breguet Range Equation are found to be inadequate for predicting the performance of electric aircraft due to their inability to account for varying power requirements and thermal management complexities. To address these limitations, the study utilizes advanced aircraft sizing methods following the guidelines provided. The methodology incorporates conceptual design stage analyses of wing and powertrain sizing, energy source sizing, weight predictions, thermal management, and power off-takes. Practical examples of electric aircraft design are provided to demonstrate the application of these guidelines. The results, which are repeatable using the information and open-source software provided, highlight the potential for different assumptions to lead to more optimized solutions. This paper provides crucial metrics and insights beyond common specific energy or power-to-weight ratios, offering detailed information that both aircraft designers and component technologists can use to develop technology solutions and optimize aircraft designs for sustainable aviation by 2050.@en