Minecraft – How to make a zombie pigman farm

My approach to overworld farm was:

1. Choose location that corresponds to a Nether location next to enormous lava lake. Otherwise you'll spend ages carrying the lava. Place one nether portal, get a big supply of buckets.
2. Build a very tall tower, then three rather long walkways with 1 block spacing between them. Put some Lava blocks in the gaps between them (some 2 blocks apart) so that you get two enormous walls of lava flowing down.
3. Remove lava sources and quickly pour water as soon as the lava starts "dropping". As result you should end up with two enormous walls of cobblestone.
4. Cast your portals, level by level of blocks: first cobble or whatever to form the casts, then fill with lava, then cool with water into obsidian, then build another layer.
5. Remove cobblestone. Light them up, add trapdoors, add grinder infrastructure (you may need to dig the hole for the drop trap).

But that's more than 2 days of work unfortunately but I'm afraid you won't do much better for overworld survival...

Anyway, some highlights:

• You'd be hard-pressed to find lakes of lava big enough to produce enough obsidian in the overworld, plus you won't need the diamond pick and ages to dig all the obsidian for the portals, just a plenty of iron for the buckets. Casting a row of blocks for the portal like that is quite speedy, and lava is plentiful in the Nether, one step away from your construction (through the portal).
• Dispensers with TNT may be helpful in all the cobblestone removal.
• One bucket of water is all you need, OTOH you'll want a plenty of lava buckets, and to create a secure approach to the lava lake. Ghasts will be your worst problem.

EDIT: Some pics, as requested. The spawner while under development. I had decided to start 22 blocks above the ground, which, in hindsight, was a waste of time and resources. It would likely be much easier to dig a 22-block pit instead and start at ground level. Nevertheless, what you see:

• The brown "startup" tower on the right, built over a nearby river for safe landings, with ivy for climbing (easier to get than ladders). The "loop" on the top is remains of the construction for pouring the "walls".
• a "house" in the middle, with railway and stairs access. Storage, bed, etc. Also, infinite water source for when I misjudge pouring it and turn it to stone.
• Staircase. Both the house and the stairs are built in a "glitch", lava pouring sideways. As you can judge from numerous holes and gaps, the process of "pouring the walls" isn't perfect. Many holes appear, but that isn't that much of a problem, you can seal them by hand as you build the portals.
• A piece of the front wall uncovered on the right, revealing a few of first portals. There are ready portals in the wall about as high as the staircase reaches.

Now, making a mini version of that.

Two gaps

Lava every third block

That forms two lava walls.

Pour water to form two walls of cobble. There will be mistakes and imperfections. Don't worry.

Create cast for the bottom of the portals. Seal any holes resulting from imperfect walls. Pour lava in Solidify. Cast for portal walls Lava Water. Repeat three times. Another layer of portal floors/bottoms And more walls.

Remove all the cobble

And light that up.

Turning that into actual grinder is left as an exercise to the reader.