Build your Bot
Create a build directory (folder) with mkdir build
Enter the folder with cd build
Go back to your Application's 'bot' page from before, and copy the token. Run echo Bot [token] > token.dat, replacing [token] with your bot's token. Alternatively, you can set the ECHO_BOT_TOKEN environment variable to Bot [token].
Build the makefiles with cmake ../
If you are using vcpkg then run cmake -DCMAKE_TOOLCHAIN_FILE="your vcpkg.cmake folder" ../
Build the bot with make (if you are using Visual Studio open echo_bot.sln and build from there)
Run the bot with ./echo_bot
You can change the executable name by modifying the project command in CMakeLists.txt
???
Profit! Mention your bot in a scope it can read & write in, it should echo back your message.

[ 50%] Building CXX object CMakeFiles/echo_bot.dir/main.cc.o
In file included from /home/anton/prj/echo-bot/main.cc:9:0:
/home/anton/prj/echo-bot/lib/websocket-beast/discordpp/websocket-beast.hh: In instantiation of ‘void discordpp::WebsocketBeast<BASE>::runctd() [with BASE = discordpp::RestBeast<discordpp::Bot>]’:
/home/anton/prj/echo-bot/main.cc:129:1:   required from here
/home/anton/prj/echo-bot/lib/websocket-beast/discordpp/websocket-beast.hh:54:32: error: no matching function for call to ‘discordpp::WebsocketBeast<discordpp::RestBeast<discordpp::Bot> >::call(const char [4], const char [13])’
             json gateway = call("GET", "/gateway/bot");
                            ~~~~^~~~~~~~~~~~~~~~~~~~~~~
In file included from /home/anton/prj/echo-bot/main.cc:8:0:
/home/anton/prj/echo-bot/lib/rest-beast/discordpp/rest-beast.hh:29:22: note: candidate: discordpp::json discordpp::RestBeast<BASE>::call(boost::beast::http::verb, std::__cxx11::string, discordpp::json) [with BASE = discordpp::Bot; discordpp::json = nlohmann::basic_json<>; std::__cxx11::string = std::__cxx11::basic_string<char>]
         virtual json call(http::verb requestType, std::string targetURL, json body) {
                      ^~~~
/home/anton/prj/echo-bot/lib/rest-beast/discordpp/rest-beast.hh:29:22: note:   candidate expects 3 arguments, 2 provided
/home/anton/prj/echo-bot/lib/rest-beast/discordpp/rest-beast.hh:128:22: note: candidate: discordpp::json discordpp::RestBeast<BASE>::call(std::__cxx11::string, std::__cxx11::string, discordpp::json) [with BASE = discordpp::Bot; discordpp::json = nlohmann::basic_json<>; std::__cxx11::string = std::__cxx11::basic_string<char>]
         virtual json call(std::string requestType, std::string targetURL, json body) override {
                      ^~~~
/home/anton/prj/echo-bot/lib/rest-beast/discordpp/rest-beast.hh:128:22: note:   candidate expects 3 arguments, 2 provided