I'm depending on Google Translate so I may not understnd the question, but I believe you have only one text frame per page. If that's the case, you can set your page margins to fit the dimensions and position where you would like this frame to be and use auto-flow to place your text.
Merci Peter de ta prompte réponse,
grace à Reverso, j'ai pu comprendre que c'était le pavé déterminé par page qui commandait.
Malheureusement j'ai déjà un pavé complet qui contient le texte explicatif.
Thank you Peter of your quick answer,
Grace in Reverso, I was able to understand(include) that it was the determined pavement by page which commanded(ordered).
Unfortunately I already have a complete pavement which contains the explanatory text.
I don't think I have anything more to offer. You should consider adjusting the master page so the margins are as I suggested above, then you can pick up the overset text and flow it as suggested.