You will need:
1) Tape Measure (metal ones are best)
2) Pen/Pencil

Where do you want your blind to be fitted?

How to measure inside the window recess
1) Inside the window recess
2) Outside the window recess
How to measure inside the window recess:
1) Measure the width of the recess from wall to wall in 3 places 
Note down the shortest measurement.(remember to label this measurement width)
2) Measure the drop by taking the measurement from the top of the recess of the window sill in 3 places

How to measure outside the window recess:

1) Measure the exact width of the window recess and add 50mm onto this for Venetian, Roller and Pleated blinds and 100mm onto this for Vertical blinds (this is to make sure there are no gaps). Note down the measurement. (make sure to label this measurement width)
2) Measure the exact drop from top to bottom of the window recess and add 50mm onto this for Venetian, Roller and Pleated blinds and 100mm onto this for Vertical blinds (this is to make sure there are no gaps) . Note down this measurement. (make sure to label this measurement drop)

Measuring tips:

Take all of your measurements in millimetres (mm) this will give you a much more accurate measurement.While measuring make sure to look out for things like window handles and tiles taking into account their position if necessary. This is to ensure a perfect fit.