Emissive Cape Portals is a core shader. Core shaders are loaded through the normal Resource Packs menu, so you switch this one on the same way you would switch on a texture pack.
Enable the pack
1. From the title screen or the pause menu, open Options → Resource Packs.
2. Find Emissive Cape Portals in the Available list on the left. If it is missing, click Open Pack Folder, put the pack file in the resourcepacks folder that opens, and return to the menu.
3. Click the arrow on the pack to move it into the Selected list on the right.
4. The pack is small, so this should be quick.
Check that it worked
1. Make sure you are wearing one of the movie capes. The glow only affects the portal shapes on those capes.
2. Go somewhere dark, such as a cave or the outdoors at night.
3. Press F5 to switch to third-person view so you can see your back.
4. The portal design on the cape should glow and stay bright while the area around you stays dark.
If the cape looks the same as before, see the troubleshooting guide. Most of the time the pack is not in the Selected list or another pack is placed above it.