Janu sirsasana C is the third in a series of janu sirsasana poses where one knee is bent with the shin folded to the inside of the thigh. In the C variation the foot is placed against the inner thigh of the other leg with the toes tucked under and the foot oriented downwards so that the heel is uppermost.
When bent forwards this pose is a hamstring stretch for the straight leg. But it is also a calf and ankle stretch for the bent knee leg.
The foot position of the bent knee leg is actually very similiar to half lotus pose. The main difference is in the position of the foot.
And so you could think of this pose as a prep or modification for half lotus pose or vice versa, depending on which one you find easier.
An option for this pose, if you really want to focus on your forward bends, is to place the sole of the foot on the floor with the knee uppermost and the foot turned out ninety degrees.
You could then work towards the final foot position by gradually working your foot in closer while lifting the heel and moving the knee towards the floor.
Another option that may help to prepare the foot and ankle is to kneel with your toes tucked under. Then turn one knee out to the side and place the foot of that leg against the inner thigh of the other leg.
Start with the toes of both fee tucked under. In the picture below I've added a twist, hower you can simply face forwards or if you wish, bend forwards.
From here, you can lower the pelvis so that it more closely resembles the janu sirsasana C foot position by placing the top of the "non-janu-sirsasana" foot flat on the floor.
From here the next variation would be to straighten the non-janu-sirsasana knee.
In this position you can experiment with pressing the heel of the foot against the inner thigh. You may find that this action helps the knee to move down towards the floor.
You can also experiment with pressing the toes of that foot into or against the floor.
When bending forwards, move slowly and stay aware of your bent knee. Use your hands to support your body both when moving into the posture and when moving out of it.
Even when you have gotten used to doing the pose, practice moving in and out of it (and any other yoga pose) with awareness. If you aren't sure what that means, then practice moving in and out of any yoga pose slowly and smoothly. That way you can stop if you feel anything is wrong.