3D CAD may be the lower-stress route to handle the multi-protrusion form of the Ex-S. For maximum masochistic experience; use sliderule and drafting projection methods from orthographic views in MSPaint.
The two quickest cleanest sources of shape data for "MSA-0011 Ext" I can offer are:
http://www.gearsonline.net/gundam/sentinel/exs-gundam/exs-gundam.php
http://www.1999.co.jp/eng/
When building,
Light spraycoat clear acrylic (various brands), let cure 1 hour to 6 months.
Wash hands and dry (repeatedly, often, between each task, after using lavatory...)
Boss folding lines (before cutting)
Sharpen blade
Cut parts (smoothly, one stroke, straightedge...)
All of this fairly basic, and covered in FAQ and "Tips and Techniques"
http://www.cardfaq.org/faq/
http://www.cardmodels.net/phpbb2/viewforum.php?f=11
If you slice at 45degree into the overlap side, keeping the part edge on the upper surface, the closed seem over a gluetab can be smoothed easier. Omit gluetabs altogether and butt-join with backing strip.
Color exposed edges (with feltip marker or such), before gluing.
I apply a film of PVA glue ("Elmers" white glue) from a scrap using bamboo skewer, then clamp tight with smooth-jawed needle-nosed pliers. Also doing most folding and manipulation with these pliers.