Take more time than usual. Look at the tape twice to be sure the number you wrote down is correct. Try to take someone with you to help measure. Take pictures from all angles to help you remember what all those nooks and crannies looked like. Take interior shots, too, especially if there are multiple levels.
When you're finished, walk around the exterior one more time and make sure your sketch matches what you see and that all your walls and stuff have numbers by them.
Above all, enjoy the change of pace and the challenge! 8)
Use a grid paper which is somewhat translucent so that you can lay a new blank over, say, the main level you started with, and then transfer key corners, set-back measurements, or cantilevered extensions from the main sketch to your upper level or even your basement. I align the two pages evenly and mark these "dots" on the clean blank. Watch out for the high vaulted ceilings inside which may portray false living areas of upper levels if viewed from outside ! On Tuesday this week I discovered a 329sf discrepancy with assessor records for upper level of a two-story and a 352 sf discrepancy in agent's number posted in his listing for this property under contract. I portrayed my lesser number quite succinctly in my sketch and proceeded to use it, added to main level, as I moved on to the grid. I did not have the desire to press the issue with the listing agent....just finish the report and disclose my "discovery" in the addendum. There is a reason why we are relevant in the marketplace ! Bottom line, it's the upper levels of the home which will involve your greater skills
Graph paper is a must, it's much cheaper than an attorney. Always measure the extreme perimeter afterwards to make sure that all those 10', 12' and 17' walls add up to whatever they should....take photos of the exterior angles....if it's 3 stories with weird angles, I have 3 floor plans before I set foot inside.....on Cape Cods measure those dormer distances from the outside first and check them again when you leave....one last tip.....if the assignment is for an AMC and you're in a hurry or it's raining, go ahead and leave......when you're back in the office simply scan in a copy of the tax card and write "INTERIOR AVM" across the top in big bold letters.....
Yes, graph paper is a must!!! And make sure to draw very carefully to scale on the graph paper with lots of dots and xs that refer to something of a different level, also note doors and their hinge side, and fences or windows or something that might indicate where an interior wall might be. When inside, do an interior wall sketch, even some measurements of some of the rooms. If the interior wall sketch fits inside your permeter measurements, less chance for error--it will also catch you if the interior doesn't fit the exterior, then you can go remeasure something on the exterior.
A trick I learned from an appraiser who has made up cards to hand out. Have an angle, take two credit cards with the long sides against each wall, that can give you the angle, which you could transfer to a piece of paper and/or your sketch. This appraiser has had plastic cards made the size of credit cards, each card has tenth of a foot on one side, inches on the other and a protractor in the middle. When the two cards are put against the wall--they tell you the angle of the wall. He was going to sell them, had thousands made, nobody would buy them so now he gives them out as an advertising tool and to students in his appraisal classes.
That credit card thing sounds interesting. So, you have an angle, but then one is trying to draw a very, very small line segment on a piece of graph paper while preserving that angle as measured. No doubt, there are still things I have not yet learned about my APEX drawing program, and one might be "taking off" from a point at a certain measured angle. However, the other ol' geometry trick I employ is simple rise-over-run, or ...how many feet out, how many feet over, to get my line segment extended to the next turning point in the perimeter line of this exterior wall. APEX very easily enables one to draw any angled line as an out-over process so long as those two lengths (numbers) pass through the obligatory right angle.... out there "in space". Just a thought, it works for me using APEX.