how does awgn works

if h is normalaized to have 0dbw then after x=awgn(h,5);

1-what formula should i apply on x to see 5db;

2-is 10*log10(norm(fft(x)^2/length(x))) a coorect way for clculating power ?

3-given a signal and certain snr value what calculation should i do to make this signal have this snr ?

Thanks in advance