Hi, yes get all the old paper off then repair any bits that need it. Hairline cracks are ok to leave but not any others as they will show through the paper. After this size the walls as usual so that when you paste the walls it doesn't dry too quickly which will make the paper come away from the wall especially the seams. Remember the finish will only be as good as the preparation. Any lumps left on the walls before papering will probably show through after papering.