If you have decided to file a S.45(2) election with respect to a change of use of your property from personal use to income producing use, you should file a paper return with CRA (instead of Netfiling) attaching to the return the details of the S.45(2) election that you are making (address of the property, date of the change of use, etc. and signing and dating this attached S.45(2) document - keep a copy for your records).
Here is a document that summarizes what is involved with the election.
http://www.mnp.ca/en/posts/changing-your-principal-residence-into-a-rental-property
Note the section that states that the "election must be filed in the return for the taxation year in which the change occurred. Canada Revenue Agency will accept a late-filed election if CCA has not been claimed, but penalties may apply".
