how to take components cos theta and sine thera correctly in FBDs

You will have an angle first, which must have its two arms and the angle between those arm will the angle with respect to which you are going to get components on the plane.
Suppose angle is somewhere in the mid position say around 50 degree. So now you will have one arm on horizontal and one arm at 50 degree to the last one.
You can only two components of it in two directions first one you can go for horizontal axis and other one will be vertical axis so now, if you are measuring angle anticlockwise from the horizontal then, horizontal component which is just adjacent to the horizontal arm of the angle will be the cos component and vertical component which is away from the angle will be the sin component.
Draw an angle first..make two components of it.
Close the triangle.
Put names to the three sides, and apply sin and cos of the taken angle, you can understand that the horizontal one is cos component and vertical one is sin component.