There are several online sources to find a design that you like. As to the actual construction though, this will take a bit of planning to ensure the compatibility of your addition to your existing deck. The actual rafter layout is something that you can search for online. The critical thing to watch for is to ensure that your roof loads are carried directly to your foundation. You cannot have a post carrying a roof load just sitting anywhere on your deck surface. You cannot point load a beam either.You may have to add additional concrete pad footings and columns to carry these new loads. Ensure too that the lintels carrying the roof structure meet the minimum size as required by your Provincial Building Code. Most Gazebo design books will specify a 'nice looking cedar beam' from post to post to carry the roof. The problem is that the Building Code has no provision for cedar lumber as a structural member. You will have to use Graded dimensioned lumber (i.e. 2 x 8 or 10 SPF ) and then face the built up beam with cedar fascia. You should use pressure treated lumber for any of your structural components and face them with cedar afterwards. If you NEED to use cedar lumber for the structural components of the building, often your local Municipal Building Inspector will require you to have a Structural Engineer 'sign on' for design and review of your proposed structure. Get a building permit for your work regardless of which way you go. This will help ensure that your design and construction methods will meet at least the minimums required under the Building Code. This will also ensure that if something were to happen in the future where someone is hurt due to failure of the structure, your insurance company will cover you for it. If you don't have a permit and the inspections, your insurance company may find this as a means of avoiding covering you should some unforeseen accident happen. Good luck in your search for designs.
Have a great Day! Mike