It's posable but probably not by this weekend. First you have to find a non-filtered non-pasteurized beer (not that hard). Then pour most of the beer into a glass and drink it. Sanitize the mouth of the bottle and a jar. Swirl the beer around and pour it into the jar. Next make a very small starter, maybe a cup or so, and feed it to the collected yeast. Once the starter has run its course, chill it, pour off the spent liquid, and repeat with a larger starter. Keep this up till you have the amount of yeast you need.