First she have to apply for tourist visa valid for three months.
You may send her an invitation letter, copy of your PAN card and a return ticket valid for 90 days.
Next step is to visit the office of the Marriage registar in Bandra (E) in Mumbai.
All marriages with a foreigner has to solemnized under the Special Marriage Act of 1954.
Fill up the marriage intent form jointly.
For this they require your age proof, (Passport, Birth Certificate) your address proof ( Passport, Ration Card, Voters Identity card) and Martial status. If any one of the spouse is divorced then the divorce decree along with the judgement copy has to be submitted. If the divorce decree is in any other language other then English get it translated and attested by your counsulate/embassy. Passport size photographs are also to be pasted on this notice. All the documents has to be photocopied at the time of submission, photocopies has to be notarized. You have to pay a nominal sum as fees for which a receipt is given.
Then after two weeks visit the Marriage Registars office for the actual date ot marriage. At this time have all your original documents ready so that the Marriage Registar himself can verify. After verification of the documents the registar will give a date for the marriage. The date of marriage will be stamped on the back of the receipt. You can also say that you will show all the original documents on the day of the marriage. You can choose your date of marriage minimum one month from the date of submission of the form to maximum three months.
For the marriage itself you have to have three witnesses. All witness should have their passport size photographs ( 3 copies) along with proof of residence. You should also carry 3 nos of passport size photographs. The marriage itself is solemnized in front of the marriage registrar where in the husband and wife has to take the oath. The marriage certificate will be issued only after 15 days.
Since your wife will be here on a tourist visa, you have to get her visa converted into short visit visa from a tourist visa (valid for one year).
For this you can visit the Foreigners Regional Registration Office (FRRO) at Mumbai.
However the authority to sanction/approve this would be The Director (Foreigner), Ministry of Home affairs, Visa Facilitation Centre, Jaisalmer House, 26, Mansingh Road, New Delhi - 110 011.
After approval you may plan to convert it into PIO since any foreigner married to Indian citizen is eligible.