Hey there.
Good job at articulating clear questions that can be answered easily!
Before answering your questions in specific, let's talk a bit how hitsounds work in osu!:
osu! uses 3 different sets of hitsounds that are referred to as "soft", "normal" and "drum".
Each of these sets consists of 4 hitsounds, the so-called hitnormal that is played on every note that has no or only 1 additional hitsound and the additional hitsounds "whistle", "finish" and "clap".
When skinning or working with custom hitsounds you need to supply hitsounds relating to these terms.
The pattern for naming these is the following:
<set>-hit<sound>.wav
So for example normal-hitnormal.wav or drum-hitclap.wav
1. In order to use hitsounds from a skin for a beatmap you need to extract the related audio files from the skin first.
To do that, enter the Options and scroll down to click "Open current skin folder" while you have the desired skin selected. In the skin folder you should be able to find hitsound files according to the naming pattern mentioned above.
You can copy these and insert them into the folder of your beatmap and osu! will recognise them and use them in place of the default hitsounds.
Note that it is considered poor behaviour to upload hitsounds from a skin without asking the creator for permission to use them so if possible seek them out before you upload your beatmap with the hitsounds in them.
2. I'm not sure what instructions exactly you tried to follow, was it
this one?
Essentially osu! recognises the hitsounds in the beatmap folder if the format is correct. That means if you have a hitsound which you want to add, you simply rename it to the hitsound it should replace.
If you need to add more hitsounds you can work with different hitsound sets by adding numbers to the filenames such as drum-hitnormal2.wav and setting the sampleset in the timing tab as shown in that guide.
Feel free to ask further if something is unclear or you need more information :-)