You may consider warm up with some loosening exercises followed by the Surya namaskar [Sun-Salutation] (at least 5 sets of 12 exercises synchronized with recommended breathings). Then you can consider to do following asanas before attempting the down dog pose:
(i) Tadasana (Mountain Pose) to spinal straightening effect and relaxation.
(ii) Katichakrasana (waist rotating pose) for giving twisting effect to the spine.
(iii) Padahasthasana (standing forward bend) to prepare for down dog pose
(iv) Ardhmatsyendrasana or vakrasana (Half Spinal Twist) to tone up spine as well.
(v) Yogamudrasana (Psychic Union Pose) or Jaanu-shirshasana (Head-to-Knee Pose) plus Passchimotanasana (Seated Forward Bend).
Now since the flexibility of the lower back shall increase gradually, so, try to attempt the down dog pose which you shall now be able to do slightly easily.
Thereafter, it is recommended that you do backward poses like bhujangasana (cobra pose) and shalabhasana (Locust Pose) and then you can do setubandhasana (bridge pose).
I also recommend you to do quick relaxation after each pose or round of suryanamaskar. At the end, do not forget to do deep relaxation technique. It is mandatory.
If you have any spinal injury, lumbar spondylitis or slipped disc, I recommend not to do any pose involving spine or back muscles, including down dog pose. It may harm your instead of any benefit. Please look at the following video to understand the real meaning of asana:
https://www.youtube.com/watch?v=gtitq4UhSro
You are free to continue with your questions if any on the subject, I will be happy to answer.