you have to go through VFS(3rd party) for applying to CIC. this is the link you can find the city and submit accordingly... they charge a nominal fee of 600rs for a application i guess...
http://www.vfs-canada.co.in/visaapplication.html
see the worker section of the application you might be able to get the checklist..also regarding medicals its purely depends on the CIC...sometimes they dont ask you to redo medicals...but sometimes they will ask...since you have done immigrations medicals for PR(if its valid that is not more than one year) they tend to be ok with that medicals... if they still need you to redo medicals they will send you the forms once you apply...
Step1: follow the checklist and fill the application form, with photos, copy of work permit also possible attach the receipt of your earlier immigration(PR) medical test receipt along with your passport..
Step2: they might issue the new Visa or if they think you need a medical test they will send you forms and you go to a DMP near your city and they will send the passport with VISA
without medicals processing time would be less than a week... with medicals it will be 1-2 weeks..
also you are saying you will be staying for 45days...my advice is get yourself submit the application as you land get your VISA stamped at the earliest...so they might not ask you for redoing medcials...
the main reason for asking you to do medicals if its more than 30days is to make sure you dont get anything from india and travel to canada