The primary issue for most people is protecting the knee, which can be very vulnerable in Padmasana (Lotus Pose) and its variations. Padmasana requires a fair amount of flexibility in the hips. When the hips are tight, the knee ends up taking too much pressure and can become strained or injured. Anatomically, the hip is a ball-and-socket joint, which means it is designed for a wide and varied range of motion. The knee, however, is a hinge joint, not designed for the same kind of actions. Therefore it is crucial, when preparing for Padmasana, to develop flexibility in the hips so that the knee isn't compromised.
My suggestion is to begin with a variety of externally rotated standing postures, such as Virabhadrasana II (Warrior II Pose), Utthita Parsvakonasana (Extended Side Angle Pose), and Utthita Trikonasana (Extended Triangle Pose), as well as a series of supine and seated hip openers. On the floor, you could begin with "thread the needle" on your back (which puts the least pressure on the knee but provides a deep stretch), followed by Janu Sirsasana (Head-to-Knee Forward Bend) or an Ardha Matsyandrasana (Half Lord of the Fishes Pose) variation, and then double pigeon or Gomukhasana (Cow Face Pose). In addition, I would warm up your knees a bit further with a Marichyasana (Pose Dedicated to the Sage Marichi), probably B, since you have facility in Ardha Padmasana.
While these poses can help prepare for Padmasana, it is very important that when you actually practice the pose itself, you also practice ahimsa, or nonharming. Pay attention to rotating your leg from as high in the hip socket as possible, and do not ignore pain in your knee if it does occur. Also consider working with a teacher who has a sense of your body and practice, and who can help you in a more direct way with the details of alignment. Padmasana takes time and patience and is never worth forcing.