Flat aluminium foil is pretty useless for constructing a cup for boiling water but can be done if the aluminium foil is formed into a cone shape wrapped a few times and the last edge sealed with some duct tape. The tip of the cone needs to be folded over as well and secured with duct tape. You then have to arrange the heat source is applied to the opposite side of the aluminium cone so that it doesn't burn the duct tape. Overall the size of the formed cup ends up being really small, it is difficult to fabricate even in good conditions and always has the chance of everything going horribly wrong. Not really recommended.

Even folded up premade Aluminium pie and pudding dish are tricky to use but are certainly a major improvment over flat alumiumium foil improvised folded cup.

http://www.buyrite.co.uk/AluminiumFoilContainers_001.htm