I bought a Singer sewing machine at a yardsale and it did not have a users manuel with it. I am not sure ow to work it. I tried to put the bobbin in tonight and I got it out but could not get it to go back in. it is an older model I am sure, the only number I could find on it was 621B. Could you please tell me where I can find a free user manuel? Thank you very much. Gwen Stephens