Attach the base to the mast.
Resist your impulse to assemble the whole foil right away because it is much easier to fix only the mast to the board first. The front screw is the more important one as it has to resist all tensile forces and therefore has to be long!
The next step is to mount the fuselage to the mast – comfortably at waist height – and afterwards the wings to the fuselage.