Option 1: You are lucky and have trees (big enough) in your garden
If you have 2, 3, 4 or more trees in your garden (with a diameter of 15cm or more), then you are ready to go. Get yourself a slackline , follow the instruction of the manual to set it up and enjoy!
Nevertheless, here we can also have the case of trees to far apart from each other. In this configuration, you want to get a slackline long enough (A classic line XL should be enough in most cases). And, if you want to make it easier to walk the slackline, you can use a slackline frame, chairs or any object strong and big enough that can be place bellow the slackline to adjust the length and make it easier to walk. Make sure that it is high enough so that the slackline is under tension on it.