Whether you are getting an exterior or interior BPO, then your job is just to value the property. A CMA on steroids, so to speak.
For an exterior BPO, you drive out to the house, use your digital camera to take the photos that they ask for in the BPO order (front, sides, maybe back, street view(s), street number on property, maybe the street sign). If it is an interior BPO, you have to get into the property (they always list a contact name/number in the BPO order) and take photos of each room and any damage.
You go back to the office, download the photos onto your PC, go to the BPO companys web site, log in, select the BPO they want you to complete, call up the information entry form. You also log into your MLS, get the public record information on the property, search for any MLS records on that property (expired, sold, active, cancelled, all categories), and enter the property data into the BPO form. You may have to answer some other questions (number of properties on market, value range of on-market or solds, details about any prior MLS listings of the subject property, etc.
Then you search the MLS for comparable properties. Depending on location (urban, suburban, rural), the BPO company asks you to look, for example, 1/2 mile or 1 mile or 3 miles around subject property. I look for all active, UAG, and sold properties (last 3-6 months ONLY) with +/- 1 bedroom, +/- 20% of subject living area, +/- 10 years for recently built or +/- 25 years for older comparable properties, +/- 20% lot size if necessary, perhaps same style (ranch, colonial, etc) if necessary. But, you find and follow the BPO company's guidelines in terms of these items.
You enter the comparable property data into the BPO form. Then you adjust the comparable properties to make them similar to the subject property. For example, if the comparable has an extra bedroom when compared to the subject, I SUBTRACT $10,000 from the COMPARABLE'S sale price. if the comparable has one fewer bedroom when compared to the subject, I ADD $10,000 from the COMPARABLE'S sale price. You do this for each comparable.
You save an image of the front of each comparable property. You upload the comparable front photos and your subject property photos into the form.
Then the toughest part. Keeping in mind any directions from the BPO company as to timeframe (e.g. they want a 90 day sale period, or 120 day period, or 30 get-rid-of-this-REO period), you review the adjusted on-market value for ACT properties, and the adjusted sale value for SLD properties, keeping in mind how long they were on the market vis a vis your requested valuation timeframe, and you come up with the most reasonable listing price and probable sale price for the subject property. Enter it into the BPO, and send it off to QA!