0: First, You have to get 1st birthday skin pack or Cute Kitty HD Skin pack on Marketplace.

1: Download the skin pack.

2: Rename the end  file extension to .zip.

3: Open Appdata (Win+R, then type appdata) and go into Roaming (since it's a newer version of bedrock now).

4: Open Minecraft Bedrock, then premium_cache, then open skin_packs, and delete everything inside.

5: Extract your .zip file, then drag the "Alleis Skin Pack v1.3.6 CuteKittyHD" (If you get Cute Kitty HD skin pack) or "Alleis Skin Pack v1.3.6 1stBirthday" (If you get 1st birthday skin pack) folder into the skin_packs folder (if you're using WinRAR, open the folder first, then drag the folder inside the first folder into skin_packs).

6. Have Fun!

(Thanks ClackL_Gaming. Most of this guide is quoted from his comments.)