Apache allows you to declare an intermediate TLS certificate along with your regular certificate in your configuration, but many web servers only allow you to provide one certificate option. Like Nginx, go, or heroku.
In those cases, you will need to concatenate the entire certificate chain into one certificate file. This may sound daunting, but the process is very simple. Let me introduce cat -- concatenate and print files.
cat is normally used for printing files, but in this case we actually want to concatenate files. Below is a simple example on how we can do this:
cp example_com.crt example_com.chained.crt
cat AddTrustExternalCARoot.crt >> example_com.chained.crt
cat COMODORSAAddTrustCA.crt >> example_com.chained.crt
cat COMODORSADomainValidationSecureServerCA.crt >> example_com.chained.crt
To shorten it, we only need to use cat once:
cp example_com{,.chained}.crt &&
cat AddTrustExternalCARoot.crt COMODORSAAddTrustCA.crt COMODORSADomainValidationSecureServerCA.crt >> example_com.chained.crt