How you set up a vocoder unfortunately depends on your DAW and your vocoder. There are some vocoders that vocode the left channel with the right and some that work on a side-chain, that is to say that you send one audio source to channels 1 and 2 on the track with the vocoder on it and the other to channels 3 and 4.
For free stuff, I like MDA vocode and MDA Talkbox. They are examples of vocoders that process one channel of a stereo track with the other.
Then there's ReaVocode. I don't like that as much, but it's an example of a side-chain vocoder. Even if you don't use Reaper, you can get their VST plugin pack for free.
The next thing to bear in mind is that the vocoding will only work for as long as both sources are active. So let's say you want to vocode yourself with a dog howl or a lion roar. As soon as the monster sample is finish, the vocoding will stop as well. So you'll either need to line up a bunch of similar samples in sequence such that the joins aren't too noticeable when the vocoded output comes through or to oop the same sample in the same, smooth way.
The thing to remember is that you'll need some highs in the sample so that the sibilants of whatever you're saying carry over. Some vocoders allow you to add noise to the modulator so that you have the tone of your sample with some breath as well. Waves Morphoder is one such.
My favourite tool for things like this by far is Morph by Zynaptiq, but that costs something around the $150-200 mark. It's very good at imparting the characteristics you want while preserving a clean and easily understood sound. It's not exactly a vocoder, but it's definitely a very close relative and produces similar results in a lot of situations.
Hope this helps.