How to take a correct measurement for a bikini by yourself ?

You need a measuring tape for that.
(It can be bought in any sewing shop or hobby markets or can be easily found at mom or granny’s place)

The tape must be held horizontally.

Chest girt must be measured on projecting points – shoulder blades and nipples
Waist must be measured in the thinnest area
Hips are measured in the most projecting part – buttocks

You see! Everything is easier than it seems :)