I do it to save sinners, for the Pope and to make reparation to Jesus. Same reasons the Fatima children did it, having been asked by Mary to do this.
If I get personally sanctified in the process, that’s an added bonus granted to me by God’s mercy. I don’t offer up stuff thinking of myself, it’s for the poor souls or for some intention or just to make reparation to Jesus for man’s sins.
You just say, “Jesus, I offer up (describe whatever pain, annoyance, disappointment, grief, other bad feeling or happening you are offering up) for (describe what you’re offering it up for)”
Or you can skip the description and just let him apply it to whatever.
Then you try to bear whatever it is without complaining or being upset.
Jesus, I offer up my bad grief and anxiety to help poor souls in Purgatory.
Jesus, I offer up the fact that the traffic is going so slowly and will make me late to church when I wanted to be on time. I offer up my frustration. I’m trying not to be impatient. Please help me.
Jesus, I offer up the fact that I really wanted dessert tonight and bought one at the takeout and then when I got home discovered they didn’t put it in the bag and it’s too late to go back now because they’re closed. I’m really frustrated and unhappy about this, Jesus, because I paid and everything, but I will try to bear it patiently if you could please use my sacrifice to help someone else like a sinner who needs help. Dear Jesus, this is a small thing I know but it really seems like a big deal to me right now because I had such a bad day and was really looking forward to that dessert. So I figure maybe it is worth letting one soul out of Purgatory. Could you do that please Jesus? Thank you.
Et cetera