Best frame pump ever and may be the most functional . durable . bike thing to ever come out of France. It's been around for more than 34 years . nearly unchanged . and no need to.
It typically goes under the top tube . between the head tube and the seat tube. Can also go in front of the seat tube between the top tube and bottom bracket. Clever people find ways to mount it other places . too . but...baby steps.
Order the right size
("Gaps" below refer to the space BETWEEN the tubes.)
No. 2 fits tube gaps 42cm - 46cm
No. 3 fits tube gaps 46cm - 52cm
No. 4 fits tube gaps 51cm - 56cm