On the left side of the popup there is a category box, expand "Connect" and click on "Servers" sub-page.In the "Nickname" box write your bot's account username you created in step 2 and leave all the other boxes blank. When you will successfully install mIRC, launch it and you should get a popup screen which looks like that on the picture.You can name it whatever you want but I would advise adding the "bot" bit at the end so people will know that it's a bot and not a real person chatting.
You will also need another Twitch account to link the bot to.To start off, you will need the program itself which can be downloaded either from the official site or mirrors provided there for free.