<p>I am making a DIY 3D printer based on the prusa mini. So, I doubt is if RAMPS can only control 4 stepper motors as it has slots for 5 stepper motor slots and one will not be used in my case, if I can in what order should I connect the drivers and also ho will Marlin figure out which slot the extruder is connected? Sorry for the lack of knowledge if fit is really obvious  :|</p>

Generally, a timing belt is a complicated device and many things depend on its internal construction and materials (it may be damaged when bound in wrong direction, or when cut, and when overtighten of course, etc.). There is also physics and math applicable, based on
Mersenne's laws
.
Some vendors provide calculators (online or as phone apps), which can calculate tension (force in Newtons or lbs) or the frequency (Hz). Therefore often the advice is to tension the belt until some (bass) sound is present - and professionals would tune belts with a sound tuner. There are also hints that belts should be possible to connect with fingers with slight or significant pressure (so not consistent). There is also visual guideline: when you slowly move the carriage with hand, the belt should remain straight. (Slowly, because belt is elastic and may behave different when moving carriage stronger and faster against friction of pulley.)
I would suggest to read
this article on 3dprintingspot.com
for many practical suggestions.