There is yet no MakeToolchain in Conan, hence this approach is more "manual" than usual, but still straightforward.
For now, the simplest way of consuming a package is this:
# install the dependencies
conan install --requires=nlohmann_json/3.11.3@ -b=missing --deployer=full_deploy
# install path of json is full_deploy/host/nlohmann_json/3.11.3/include
# Makefile is already adjusted to include this path
# now build the project
make -j