That depends on the language you want to fuzz. A good general introduction and hands-on "course" for C/C++ is https://github.com/Dor1s/libfuzzer-workshop. If you prefer Java and just want to get a feeling for how concrete fuzz targets can look like, take a look at the Jazzer examples at https://github.com/CodeIntelligenceTesting/jazzer/tree/main/....