You can obtain source to various MT4 Currency Meters
and discover the basic principles.
In a nutshell, the starting point (and there are many refinements
after this, to make it really usable) is to examine each of the
major currencies, observed in Currency Pair relationships.
So XXX is observed with UUU/XXX and XXX/YYY and XXX/ZZZ
and so on. I call this a "currency cluster". These individual
observations of "real pricing" where the Currency participates
in the Pair, provides data which when normalized and properly
processed, yields a "relative strength" Index for the Currency
XXX which might be USD for example. The "strength" is factored
out of the various observations as XXX is priced against the
other currencies in its "family".
By aggregating observations of XXX in all of these participating
pairs, in the "cluster" or "family" we can develop a relative
strength index for Currency XXX.
Like I said, starting point source code is available on the
net but to make a usable facility and not a "toy" requires a
lot more work. Better to buy something already done

Or, as it appears, you can do all the R&D and coding work
yourself "for free" ... One thing I've learned, is that Forex
traders are cheap, mostly hobbyists, instead of real businessmen
who are developing a trading operation. (No offense, just fact)
Good Trading, and Good Luck !
HyperScalper