Class two composites. We just keep banging on about them. It's for good cause, once you're out of Uni into the big wide world you will be doing a lot of them! So it's best to have a few techniques up your sleeve to give your patients the best outcome you can.
3 ways to Round your Marginal Ridges
1. Choose the Correct Sectional Matrix
Sectionals have an inbuilt curve to help shape you marginal ridge. If you place the right size sectional it's going to naturally assist by placing the contact in the right place and taper in at the occlusal. I try to get this right but don't always find it works.

2. Use a probe to Shape Prior to Polymerisation
A simple effective way is to just place the composite in the right place! Obviously. A great way to do this is to use a probe to remove excess.
Simply place the probe in the middle of the matrix at an angle and draw it out, along the matrix to the buccal then repeat for the lingual. Make sure you move from inside out to prevent leaving an open margin at the edge.
3. Use an Orange Disc
This is fantastic and reliable way to shape the interproximal and marginal ridges of your class two's. So useful in fact, that I use one as a routine on every class two.
Place the disc on the mandrel backwards (I use a disc this way 80% of the time). Draw the disc in from the side and over the marginal ridge. Simple, quick and predictable.
If you're using a tofflemire, be careful. They produce a very high contact (if one at all) and rounding the marginal ridge with a disc might leave an open contact! Yet not polishing it leaves a sharp edge - best to avoid it altogether and go for the sectional instead.