An easy method would be to use two bits of string with weights (eg shackles) attached as plumb bobs (two are needed for the mast measurement). You also need a metal tape measure, a chair/stool and a helping hand.
This method assumes the ground is roughly level.
To measure the draft from your nominated waterline, stand on the stool and drop the plumb bob adjacent to the centre of the keel
on the waterline. mark the string where the shackle hits the ground.
Repeat on the other side.
Halve the distance between the two marks and measure this length, remembering to include the shackle.
Measure the distance from the bottom of the keel to the ground.
Subtract this off the plumb bob measurement and you have your water draft.
For the mast height extend a metal tape measure on its side (it will not sag significantly this way) over the 'reference' point, so that it overhangs the boat on either side (easier than trying to find a long pole or board).
This is where you need two people.
Drop a plumb bob either side just skimming the side of the boat and mark the string when the weight hits the ground (simultaneously on both sides so that the tape measure is not changing position).
Average the measurements of the two bits of string.
Subtract the waterline plumb bob measurement off this.
Add your mast length to your ref point and you have your air draft