someone knows a tutorial to make a costume hairstyle usable in character creation?

Is there a chargen file in your documents override folder?
If it is, download leliana-cg and put it in your override. Then open your chargen in notepad, find <human_female> with a bunch of lines starting with <resource name="hf_har....>. Scroll down to </human_female> and above it add one or all of these lines

<resource name="hf_har_hb7704a" cut="1"/>
<resource name="hf_har_hb7704b" cut="1"/>
<resource name="hf_har_hdt2b" cut="1"/>

First is for the hair on the picture on the mod. Second is for the second hair + iconic braid. Third is for super long ponytail. 

If you don't want Leli's face to look like that, then go to the leliana-cg files and delete the two .mor files. 

And then you should be set to go. 

Note: I refuse to be counted as responsible if your computer bursts out in flames from this. So be careful.