Pick two or three sentences from a passage by a chosen author. Add a new sentence between each pair of existing sentences, then further sentences in the new intervals as they become available. Continue the process until the passage has attained the length desired.
Warren Motte has used the convenient term 'larding' as an equivalent of 'tireur à la ligne,' the name given by Jacques Duchateau to a procedure that is his specialty.
"Line stretching" refers to the 19th-century practice of paying magazine contributors (such as Alexandre Dumas) by the line -- a practice that encouraged them to stretch their material to a maximum length.
Duchateau describes the method in 'Atlas de litterature potentiale' (Gallimard, 1981): From a given text, pick two sentences. Add a new sentence between the first two; then two sentences in the new intervals that have become available; and continue to add sentences until the passage has attained the length desired. The supplementary sentences must either enrich the existing narrative or create a new narrative continuity. (It is permissible to start with more than two sentences where appropriate.)
p. 163 of the Oulipo Compendium (Atlas Arkhive Six, London 1998)