This is Moderator Phileas Fogg, I do apologize for the late response. However, I checked what you have requested and unfortunately I couldn't find any detailed explanation on how to customize a HUD. Therefore, I will be giving you a quick and summarized steps in how to achieve customizing your HUD successfully.
First of all; I highly recommend you to download " TXD Workshop " by googling it. Afterwards, when you have done extracting it into your preferred location. Open this file by "Right clicking" and select Run as administrator:
Once you have done opening the file, a pop-up dialog will open which will appear like this:
As you can see there is "Open TXD" just click on it and another dialog will be opened. Go to your GTA Directory folder --> Models --> "Hud.txd"
Some details will be showed in the second column. Like this:
As you can see the one which I am already selected "4th box" is your Crosshair "THE AIM". Besides that, if you kept going down till the end just go up by 1 and you are going to see your "FIST ICON" where you can change it to the one you already downloaded.
Use your "IMPORT" to get the designs which you have downloaded being applied in your GTA folder.
Keep in mind that we are not responsible for any corruption through your GTA files so I request from you to double check everything which is positioned in the right place and then select "SAVE TXD". Afterwards, close the workshop and enjoy your game.