To unlock the legendary Beasts in Crystal, you must go to the Burned Tower to start.
1) Take the stairs down to the basement. You will find all the beasts.
2) Approach them and awaken them. They will be then run away
3) Raikou and Entei are located in any route in Johto. When you encounter them, they will battle you and try to flee. Try not to kill them as you will ruin your chance. You will eventually encounter them and catch them.
4) As for suicune, he is located in Tin Tower (you will see it standing). This is your only chance and you won't be able to encounter them again.
*Be patient with finding them as it will come to you. They are much harder to catch than any other game so try using the Master Ball on your favourite one - altough quick balls work great too.
If you're still having trouble, take a look at this website. Hope I helped! :)