Let me give you an example.
Load the cherry tree model from the content browser / asset browser (depending on what version of C4D you have). It comes with a material for the bark and another one for the leaves. Copy both materials (Edit/Copy in the Material Manager).
Now go to the Corona menu and execute "Convert Scene...". This command does exactly what it says, it converts the C4D materials to Corona materials. But these materials need some work.
Let's start with the bark. Here, the conversion ran almost perfectly. You just need to adjust the glossiness value of 50% to something less wet, like 20%. But that's up to you.
Now to the leaves. Here we will have to do a little more work.
Let's start in the base layer. First thing we notice is a black "Layer" texture. When we open it, it's empty. The Corona material converter has been screwing up C4D layer shaders as long as I can remember. I hope this gets fixed one day. Until then there's a "workaround". We did not copy the original materials for nothing! So it is time to "Edit/Paste" them.
Now that we have the original materials back, we open the C4D leaf material and "copy shader" the intact layer from there and paste it in our Corona material.
The bump channel was converted correctly, we leave it as it is.
The last thing we need to adjust is the opacity. We go to the opacity channel, click on the texture and then click on "Layer Set / Select". We need the alpha channel of the texture, so check that box and click "Ok".
As a bonus, we can now add Translucency, which will often improve the realism of a plant rendering greatly.
Go the the General channel and check "Thin Shell". Now we can activate "Translucency". Increase "Value" to 50%. Copy the color texture from the base layer (it should even still be in our copy buffer) and paste it in the Color texture slot of "Translucency". I often add a little yellow to the texture here, but that's up to you, as much as tweaking any other setting along the way.
Hope this helps.
Michael