Lay out sheets of card stock to form a rectangle the size you want the final stencil to be. Count how many rows and columns you need to form that size. Gather the card stock back up.
Draw straight pencil lines on the original image to divide it into a grid containing the right number of rows and columns. Make sure the proportion of the rows' height to the columns' width matches the proportions of the card stock you are using. Label the columns with letters and the rows with numbers.
Draw the image fragment in the upper left section of the grid onto a full sheet of card stock. Label the card stock with the number and letter of the grid section. Repeat to create one sheet of card stock corresponding to every grid section.
Assemble the large stencil by arranging the card stock sheets into rows and columns according to the numbers and letters they are labeled with. Tape the card stock grid together into one large rectangle using masking tape.
Cut along the lines with a craft knife to cut out the stencil.