This is relatively simple.  Build a structural wall of 2x2's, or 1x4's (or whatever fits your shell), to keep the spa from tipping over on its side.  For the base, be sure to use 2x4's of the pressure treated variety for long life.  Be sure to use exterior grade deck screws, (galvanized or plated) to hold everything together.  (Now's the time you wished that you'd bought a screw gun or 12 volt or better DeWalt portable drill, instead of that fishing pole last summer!)  Used with caution, a standard variable speed drill with a screwdriver tip can work too. 

For the side skirting, I find that T-11 or T1-11 type of exterior grade home siding works and looks the best.  It also lasts longer than redwood, cedar, or other forms of non-treated wood.  Usually found at home supply stores such as Home Depot, Lowes, and others, it's normally sold in 4x8 sheets.

Insulating the skirting is relatively easy.  The easiest and most efficient type of insulation to use is the metal foil backed bubble wrap style of insulation, sold in rolls from home supply stores.  Simply staple it to the inside of the skirting before attaching it to the structural frame.

It will also be a good idea to provide a few small vents near where the equipment components will reside.  This is necessary to prevent overheating of the spa, and the equipment.  These vents can be as small as 2" holes, cut mid-way up in the skirting material.

It would be a good idea to also put a piece of  5/8" or thicker exterior grade plywood to cover the area of the footwell, especially if it is larger than 24" square.

The equipment pad is an important item.  Don't just install your equipment on the ground or try to do it using the supporting 2x4's.  You will be praising yourself for having a solid, steady, level base to put all the 'stuff' necessary to make your spa operate.

If you want to go first class all the way, adding an exterior grade plywood bottom to the base structure will provide the ultimate in insulation and structural integrity.

Also, be sure to make an equipment access door, (your choice on the style), that provides full access to all of the equipment.  A good size is 4 feet wide and about 2-3 feet tall.  You may have to frame it to be structurally sound.  Use standard hinges or screws to attach it to the skirt.  A horizontal door is easier for a spa technician to deal with than a door that opens vertically.   If you're a really creative carpenter type, you can build a sliding door to provide access, as used in thousands of Morgan brand spas.  It may not be necessary to add a 'door', you can simply use the entire piece of skirting as a door.  Simply attach it to the supporting frame with screws.