For this Dopero, you need long lengths of 5mm (3/16") wooden dowel. Enough for the 4 spars of 1.0DL (120cm, 48") each. They are easily cut to the lengths required with a small cheap hack-saw.
The upper and lower sails now need to be joined where the vertical spars cross.
Prepare ten lengths of electrical insulation tape, each one about 4 times longer than it is wide. Stick them by a corner onto something handy like a table edge. You can remove them one at a time as needed.
The two horizontal spars will now be lashed to the vertical spars. Firstly, just make sure that everything 'looks right', with no bunched up plastic anywhere.
After putting several turns of flying line around each crossing point, fix the joins with a generous drop of glue each. This will also prevent any shifting along the vertical spars.
As mentioned earlier, there's more kite-making on this site than you can poke a stick at :-) Want to know the most convenient way of using it all?
The Big MBK E-book Bundle is a collection of downloads - printable PDF files which provide step-by-step instructions for many kites large and small. Every kite in every MBK series.