You get the Portal Gun. Fire a blue portal with left click and an orange portal with right click, then step through one and come out of the other. Fall in from a height and you keep your speed. Items and mobs go through too, and you can see through each portal in real time, so you watch the other side before you step in.
What you do
Left click: blue portal. Right click: orange portal.
Portals stick to flat, solid block faces. Aim at a wall, floor or ceiling. Black concrete, glass, bedrock and leaves refuse a portal.
Firing a colour again moves that portal. Break the wall behind a portal and it closes.
Walk, jump, fall or throw things in. You leave the other portal facing the way it faces, with your speed kept.
Getting it
Press Play in Melty. You open any world (new or old); the first time you join a world the gun is in your inventory. You can also craft it (iron, ender eye, redstone, quartz) or type /portalcraft gun. /portalcraft clear removes your portals.
Good to know
Single player for now. It is built so friends can be added later.
You need Minecraft: Java Edition. Melty sets up its own copy of the game for this mashup (Minecraft 1.21.1 with Fabric), so your normal Minecraft is untouched. The first start downloads the game files and takes a few minutes.
On some NVIDIA graphics drivers the portal view library warns about a rare crash; updating your graphics driver helps.
Credits
Inspired by Valve's Portal. This mashup uses nothing from Valve: the art is made for it and the sounds are Minecraft's own.
Live portal view and seamless teleporting: Immersive Portals by qouteall (Apache-2.0).
Fabric Loader and Fabric API (Apache-2.0). Bundled Prism Launcher (GPL-3.0), with its licence and source link inside.
Portalcraft itself is MIT licensed and open to remixes.