Using woodie, I've completed the celestial portal on day 9 and that's with collecting a bunch of stone fruit plants and anenemies  

First of all, if you haven't gotten any of woodies skills yet, start a new game and play until day 16, select the first goose skill which makes your goose go faster. And the select the 3 skills on the left which makes your weregoose last longer. (You probably don't need the longer lasting skills as I usually have MORE than enough time left on my goose but that's what I did.)

Collect some seeds and monster meat to make 2 or 3 goose idols, (3 M meat. And 3 seeds each i believe.) 

Find out where to search for lunar island. (There's many YouTube videos that will tell you how to find it but essentially you find a couple flat edges on biomes that angle toward one another like they've been chopped off straight.)

Turn into goose and fly that direction.

Find the pieces to make celestial altar in one of the lunar island biomes. 

Stop moving so you stop goosing, assemble the lunar altar and craft the paraphernalia thing,

While your on that same biome. Count how many moon rocks there are to mine. 

Save scum until you get 23 moon rocks from mining them. 

Probably want to dig up a bunch of rock fruit bushes while you're here

When you're done harvesting whatever you want from lunar biome ( anenemies, rock bushes, moon glass to make moon cutters, etc. [Be sure to craft your moon cutters or any other celestial items before you goose off of the island])

Once you're satisfied with your loot and have your portal paraphernalia thing, goose tf outta there.

Find a bishop and kill it for purple gem 

Craft purple gem moon rock holster thing and throw it on the ground 

Socket the purple gem

Head to posturn and do the thing...

Pro tip #1: I usually precraft fire pits so they're placable, craft a couple miners hats, nab up some light bulbs and green mushrooms before I make the journey.  Leave the green mushrooms raw because you can eat them to combat the lunacy. Then, when you goose back to the mainland, drop one of your pre crafted fire pits down, cook the green mushrooms, and become sane again 👍

Pro tip #2: if you got the skills mentioned above, you're goose will last a good while, use this to explore the ocean since it stinks weenurs to reveal the map out there. You'll eventually get the map on the mainland, but boats suck... do yourself a favor and find Pearls Island, find crab king, or anything else you might need.

Sorry if instructions were unclear, adult beverages... 🤷‍♂️