A bitcoin client is the end-user software that allows us to perform bitcoin operations (sending transactions, receiving payments, and so on). When you run one, you become part of the bitcoin network.

We have chosen two common clients: Bitcoin Core and Electrum. In our example, the sender will use Electrum and the receiver will use Bitcoin Core (the most popular bitcoin client). 

For the purposes of this demonstration, I will install them on a single machine using Ubuntu 16.04.

You can install Bitcoin Core (version 15.04) using the following commands:

wget https://bitcoincore.org/bin/bitcoin-core-0.15.2/bitcoin-0.15.2-x86_64-linux-gnu.tar.gz
sudo install -m 0755 -o root -g root -t /usr/local/bin bitcoin-0.15.2/bin/*

Electrum is a lightweight wallet, which means it doesn't require you to download the entire blockchain, as we will see in the next section. Download and install the latest version of Electrum as follows:

wget https://download.electrum.org/3.2.2/Electrum-3.2.2.tar.gz
sudo apt-get install python3-setuptools python3-pyqt5 python3-pip
sudo pip3 install Electrum-3.2.2.tar.gz

Once both clients are installed, we need to synchronize them with the network.