Instructions given by Manuel de Jesus, Apostolate of the United Sacred Hearts:

The oil is prepared with the petals of one rose and Virgin Olive Oil, preferably both should be blessed.

Also, Our Mother said something very beautiful on October 23rd, and I am going to read it to you:

The roses that are placed in my altars will serve to heal many illnesses; you may eat the petals, or make tea from the petals (in other words, boil water and put the petals in the water, simmer). 

To prepare the oil: place the oil in a receptacle and place the petals in it; let it repose for 9 days. Pray a novena to Our Mother during those 9 days. Or let them repose during 13 days and pray each day; and also let them repose over an altar during 33 days, in other words, the oil with the blessed rose, in front of an image of Our Mother, praying to Her asking for the graces to bless that oil.

You may also just have the oil in your altars or in your Marian shelters, she says, “It doesn’t matter if you deposit the entire rose or all the petals into the oil, the entire rose is not necessary. The oil will be a soft balm that will have my protection so you will anoint yourself always, and the sick will feel my caresses of Mother.

This a small summary about the roses: submerge the rose or petals into the oil, no matter if it is the entire flower, preferably blessed, and placed in front of Our Mother, in our altar for 9, 13 or 33 days so the oil will receive graces in your altar.