How should I make the gain outside the bandwith of this low pass filter less than -50dB?
This is a lowpass filter frequency response specification template. Draw a frequency response keeping within the outline. For most reasonable values of the parameters, you can make a successful filter design in either analogue components or with digital processing. They are certain inviolable constraints which limit how steep a filter can be, together with its delay and ripple. A steep filter means a high delay, they can't both be specified arbitrarily.



simulate this circuit &ndash; Schematic created using CircuitLab
The filter 'difficulty' increases roughly as \$\frac{stopband dB}{transition band width}\$
A transition band 30% of the passband width is easy. A transition band 10% of the passband width is getting very difficult.