A method for preparing a resizing sleeve for lining a joining point in a pipe assembly having pipes of thicker and thinner diameter. The method includes stretching the resizing sleeve in diameter to the size of the thicker pipe in diameter of the pipe assembly, installing an installation hat according to the thicker pipe in diameter of the pipe assembly by airtightly gluing it onto the stretched resizing sleeve, and returning the resizing sleeve to the size of the thinner pipe in diameter of the pipe assembly to be lined. A suitable tool for implementing the method is also disclosed.