Here is a short video and text below of me practicing Sun Salutation A with explanations and modifications for beginners. 

1. Start with standing straight in Tadasana. Feet together, strong legs, long spine, strong abs and keep your ribcage closed. Gaze in front of you. Take a few deep breaths.
2. Inhale, take your arms up, palms together and gaze at your palms, keep your ribcage closed.
3. Exhale fold forward. You can have your knees bent as much as you need to with palms on the floor or have your knees straight and palms are on your legs. You need to feel a stretch in your hamstrings. If you can then have your legs straight and palms on the floor. 
4. Inhale, lengthen your spine and look up
5. Exhale, bend your knees, palms on the floor and step back to Chaturanga. Lower your knees if you feel that you don't have enough strength in your core and your shoulders. Keep your body straight and your shoulders above your wrists. Now bend your arms with elbows close to your body. You can lower your body on the mat or if you can keep it slightly off the floor so you work your core and shoulder strength.  
6. Inhale Upward Facing Dog. Go as high as your spine allows you to. You don't have to fully straighten your arms. 
7. Exhale Downward Facing Dog (Stay for 5 breaths). Now here you can keep your knees bent as much as you need to. Its better to have bent knees and straight back than straight knees and rounded back. Reach your armpits towards the mat and your tailbone towards the sky. With each exhale try to straighten your knees a little bit more and lower your heels as much as you can. 
8. Look forward, bend your knees and walk back to your hands, inhale lengthen your spine look up.
9. Exhale fold forward.
10. Inhale come all the way up with your arms straight above your head, palms together.
11. Exhale back to Tadasana.