Nodejs 4 is required. If yours is below that, please follow instructions here to upgrade.
npm install trelew --global
trelew
On the first run you'll be asked to generate a token.
After that, you can navigate between your boards, lists and cards just by typing their name (just start typing and press TAB for autocompletion). For visualizing available options and information about current board/list/card, use ls.
Once in a list, use add card to create a new card.
Once in a card, use ls to read its descrition, edit to edit the description, comments to read the last comments, add comment or post to add a new comment, checklists and attachments to see checklists and attachments.
cd .. will go back to the previous level.
? or help will show the available commands.
At any time you can do ls | less (or comments | less). This can be useful to read long descriptions or comments.