Normally I cannot bring my buttocks to my heels. My quads (especially the vastus lateralis) are too short and my knee is too stiff.
The most classical way is to put a rolled towel in the back of the knees or a block under the buttocks. Gradually in time you use a lower block or a thinner roll.
I made a short video how I prepare myself into hero pose. The first three preparations are explained in this video. The most poses are chosen to open the hips, lengthen the quads and/or the tensor fasciae latae. The three legged downward is to floss the nerve in the back of the knee.
Not in the video but it helps also for me tilting the hips slowly forward and backward, holding the arms next to the ears and grasping the opposite elbow while going into hero pose and focusing on my breath while slowly going down. Don’t forget to stop when something hurts !
Made with Tummee’s Yoga Sequence Builder. Images owned by Tummee, used with permission