Step 1: Download I2P http://www.i2p2.de/download.html
If your on windows, choose the first "windows graphics installer." Install. Now, navigate to where ever you installed it to, and open "start I2P (restartable)" Command prompt should come up, do some computer stuff, and then it should open ***Your default internet browser***
I2P is now started on your computer. You can close out all of your browsers, and that will not turn off I2P. You have to exit I2P from the toolbar. This is good, so you don't accidentally turn off your session of I2P. For some reason, my I2P wants to starts in IE. Here is my recommendation. Have one browser used only for I2P. So if you mainly use chrome, don't use chrome for I2P. the reason for this, is because I2P starts up slow. You have to let it collect information before you can move around in the I2P world. You want to be able to surf the normal internet on chrome, while I2P boots up on IE.
Okay, so you started the I2P program, it did it's thing, and it popped up a internet browser window with a purple background and words. If you don't want to use the browser it popped up, copy the address from the URL bar, open whatever browser you want, and paste it in. That simple.
Step 2: Ready for the "hard" part? And by hard, I mean spoon feeding a baby and they still manage to fuck it up.
Your going to need to change your browsers proxy *GASP.* Simmer the fuck down, just follow these simple instructions WHICH INCLUDES PICTURES. http://www.i2p2.de/htproxyports.html
Really folks, its copy and paste like 3 things.
After that, give it 30 minutes, and try to go to a market on I2P. At the moment, I'm only aware of themarketplace.i2p
Yes, I2P is kinda slow. More users will equal faster times.
Learn I2P people. Tor is about to tip over into the unsafe zone. Don't start worrying about it yet. Plans will be made and markets moved way before it comes to that point (I hope)
BONUS EDIT "BUT WHAT IF I'M ON TAILS?@?!?!?!"
Step 1: Turn on Tails
Step 2: Go to applications>Internet>I2P
Step 3: Success motherfucker, it's that simple.
Same "warm up" time applies to tor also, so don't get your panties in a bunch when you can't go to anything in 5 minutes.
If I fucked up something, let me know. I'm just a simple man. I put on my pants the exact same way as everyone else. Expect when I put on pants I do it surfing I2P