When baby arrives to Doha,In the airport they will issue the visa on arrival free for the baby (Mother is sponsored by Father). Father will get sms regd the visa no and details(If you registered in metrash.

once you recd this detail in the mobile, go to immigration dept and ask the form for baby family visa RP stamping.
Type the form with all the details etc and Submit the form and pay the fee QR400/ RP stamping will be done in baby passport..

Need documents
Appln form
Father ID copy
Baby Passport
Baby Photo

I done same procedure for my baby On March-13