You need to fill out the form
4890-84E.pdf
Go to the forms repository and download it. ( I tried to post the link but not allowed)
Don't forget to tick the box asking if this is for citizenship application. Then they will process it faster. Mail it to the address given. Since you have limited time to send back your RQ, you should send it by registered post. They sent back my request in 2 weeks.
The information is already in their system so it's just a printout which is sent back to you. No personal information shown except when you have used health services.
I had a similar RQ recently, suggest you also add other data even if they didn't specifically ask for it. That will speed up your processing.
My recommendation:
Proof of residence- copy of lease, utility bills. If not leased then a confirmation letter from homeowner (original not copy) confirming you lived there and an affidavit from another person confirming this. ( should be notarized)
Proof of banking- not just copies of bank records but also a letter from your bank manager confirming all the accounts you are informing them about.
Proof of ties in Canada - such as memberships, magazine subscriptions , etc etc. This goes to show that you were genuinely residing here.
Mobile phone bills etc.
As to why some get RQ and some don't there is probably no specific answer. In my case, my residence was about 1110 days , a little above required. I guess for persons whose residence is exact or near to exact of requirement, they probably check more deeper in case some date mistake or something left out.
Lastly don't panic. If everything you have sent is correct, it will take them some time to go over it but it will be processed quite soon. In my case, sent in June and DM came in early Nov. Now waiting for final letter.
All the best