it is complicated to learn pronunciation by reading the explaination. It would be so much easier if you could find a Chinese buddy show you the pronunciation, anyways, if I have to explain it by words, here you go
1. 你好supposed to be ni3 hao3, however if you say two 3 tone characters together, it takes too long, thus you say ni2 hao3 instead
2. when you say a 3rd tone character alone or if it is the last characters of the vocabulary, you always pronunciate the full 3rd tone ( well you can take it as it last longer when you say it)
3 when you say the 3rd tone character in a vocabulary, for example 我们 ( wo3 men), wo3 you dont say the full 3rd tone, which means sounds shorter
I hope it helps but the best I would suggest you ask a Chinese to show you