I'm not sure how easy it will be with a phone. I'll give you instructions, but I can also do it for you if you give me your Ravelry name and tell me which button you want.
1. Figure out which button you want and copy the link or save the image to your phone.
2. Go to "User Settings"
3. Select "edit signature"
4. Scroll down and paste the link you just copied into "Option 1" or browse your phone for the picture.
5. Save signature
6. Exit that window and then open it again. I think the button will show now. Highlight it by clicking it and click on the link button and paste your Ravelry link. That should link it to your Rav page.
I think this works. It's been awhile since I did this! It might be easier if I did it for you. I know how hard this would be on a phone.