designated activities with a job offer

hi, i have a job offer from a company that starts in April. I graduated last month and I'm to change my visa from student to designated activities. however all information I find online is for designated activities visa( job hunting) which is not my case since i possess a formal contract. in that case can someone tell me what all documents I have to gather besides the job offer form to submit to the immigration centre? and tentatively how long it takes to get the visa