Intercession can take many forms, and in the case of St. Faustina, it was already a great task given directly by the Lord during her lifetime.
In the diary entry referred to above, she was to pray that people would not fear to go to confession (the tribunal of the Lord's mercy). This is an excellent way to refer to the sacrament, for it is a place of accusation, but also acquittal, provided the person has a firm purpose of amendment. The sacrament of penance (confession) involves one accusing himself of sins, recognizing them as such, and wishing to make amends where it is possible. The judge (Christ) is merciful, and forgives. The penitent needs to have sorrow for having offended God, however. Otherwise, there is still a hardness that keeps the person from God. The sorrow must be at least involve fearing the pains of hell (imperfect contrition), but it is better if it is sorrow out of offending God because one loves Him (perfect contrition).
Besides praying for others to approach the tribunal of God's mercy, those praying must not fear to approach it themselves. Let us take the opportunity to approach this sacrament frequently.