Stretching your foreskin can take months, so don’t give up too quickly.
Every day you have to pull the foreskin back as far as possible. At first you do this when your penis is soft. Don’t use force, because then you may cause fissures in the foreskin. So do it consistently, but carefully.
If you succeed in retracting the foreskin while soft, you can try it when erect.
Another possibility is to stretch the foreskin when your penis is soft. Retract your foreskin, push it back again, but over your index and middle fingers. So you have your fingers in the opening of the foreskin. Pull, to stretch the foreskin, like you strain a rubber band. But do it gently: don’t hurt yourself.
If you don’t stretch the foreskin gently it can cause micro-tears in the foreskin. When these heal scar tissue may develop. This scarring makes the skin inflexible, making it more difficult to retract the foreskin. So be careful!
Be aware of the danger of paraphimosis.

If you are 16 or older and have not succeeded in stretching the foreskin, you better seek advice from your GP.