To answer the title question, it might be best to look for smaller projects in a language that you're very familiar with with issues marked specifically for ext

To answer the title question, it might be best to look for smaller projects in a language that you're very familiar with with issues marked specifically for external individuals to complete. Smaller scope issues should be easier and they would give you some more experience in reading code which would appear to be the issue at hand. For the larger codebases, generally you would want to get a vague idea of the architecture and then blind yourself to most of the complexity. Let's say that you wanted to work on a network based data serialization library and the issue marked out as 'contributor friendly' was adding unsigned integer support building off existing signed integer support. For that target, there's a large portion of the code that you shouldn't need to understand too deeply and there's a section that is going to be relevant to making the modification. Try to find the code that is relevant and trace the execution just by reading the code. As long as you limit the scope of your work and the codebase is somewhat compartmentalized, you can shrink the time it takes to go through large projects by a fairly large amount.