Never fitted it before but going to fit some 5" plaster coving in my house

I dont want to make a mess like the people who lived there before as i pulled theirs down with one finger so whats the process


Also two of the rooms are going to be over the max length i can buy 3.6 m so will i have to mitre cut tat the join ?

Should i use dry wall adhesive to fix it ?

what should i use to smooth oer the joints same stuff then sand it down smooth