We HIGHLY RECOMMEND using I2P over Tor for many reasons which you can see in Frequently Asked Questions (FAQ) section. Currently Tails DOES NOT work with I2P and requires too much configuration for the average user to do on each boot. We are working on Tails having I2P/custom script for Tails.
Whonix / Debian-based flavours (Tor -> I2P or Clearnet IP -> I2P)
Step 1: Run the following list of commands in the terminal of your Whonix / Debian-based operating system:
sudo apt-get install apt-transport-https -y;wget -q -O - https://repo.i2pd.xyz/.help/add_repo | sudo bash -s -;sudo apt-get update;sudo apt-get install i2pd -y;sudo service i2pd start
The above command installs secure package installation with HTTPS, gets signing keys and imports them, updates packages to recognize your knewly added repository, install i2pd and then start the i2pd service.
Step 2: In Tor Browser or your Firefox, type about:config into the URL bar, accept the warnings and press Enter. Make the following changes:
extensions.torbutton.use_nontor_proxy - true
Step 3: After setting extensions.torbutton.use_nontor_proxy to true your browser might restart to apply changes. Apply the following changes where the first is the setting you have to search and after the space is the value you want to have:
network.proxy.http - 127.0.0.1
network.proxy.http_port - 4444
IMPORTANT: Wait 2-5 minutes depending on your network in order for I2P to connect properly otherwise if you try instantly it might say that the host is unreachable.
You are now able to browse i2p websites. On Tor Browser/Firefox it is important to add http:// infront of the i2p site (example to see AlphaBay you go to http://tnaefzkcnhryeusi7hdpqujqiqmnbtah3dmjcg3gvezohunjuxbq.b32.i2p otherwise you will search it instead with duckduckgo or other search engine which you have set.
When you browse any I2P eepsite (equivalent of Tor onion hidden site) it might say in your browser on top http(insecure) or when you click to input a field it might say "The connection is not secure". You can safely ignore that, I2P is end-to-end encrypted when you brwose .i2p sites same as when you browse .onion
For other Linux flavours find the most recent I2Pd documentation. i2pd is an established alternative i2p software which is the same as the original software however it is written in C/C++ and provides faster connection, less resource-hungry on the CPU/RAM etc. After installing i2pd either from source or from repository, you can proceed to the step where you modify Firefox/Tor Browser.
IMPORTANT: Do NOT browse any other websites apart from AlphaBay i2p website. If you want to go back to normal web browsing through Tor please go through each change individually in the browser and revert it, then disable the i2pd service with sudo service i2pd stop (or restart if you want to browse other i2p websites). TBB is not configured for cross-I2P-Tor-Clearnet browsing so make sure to restart/revert changes and restart browser each time you want to visit different website. It might be a pain but it is for your own security.