i hope this step-by-step guide helps
-locate your esf folder, for most people its C:\sierra\Half-Life\esf the key thing here is it's under half-life folder
-if you already downloaded the model but it has lots of diferent files and you don't know which files you need. the file extension is ".mdl"
-determine what character the model is for or what character is the model gonna replace.
-go to ...\esf\models\player then the directory that has the name of the character (i.e. C:\sierra\half-life\esf\models\player\vegeta
note: ssj level has a different dir (i.e. gokussj)
-you will find an mdl file that has the name of the character. make a backup copy of it.
-put the mdl you downloaded in that directory. and make sure to correct the file name to match the character if it isn't done already (i.e. if you have say .. a kenshin model to replace trunks and the file name is kenshin.mdl then change it to trunks.mdl)