What the achievement asks
Craft 250 Junkyard Tinkering items in Mechagon, not counting crates. Ten points, account-wide. It also gives a decoration, the Screw-Sealed Stembarrel.
Junkyard Tinkering is Mechagon's own crafting system, run from the workbench in Rustbolt and paid for in scrap picked up all over the island.
Crates do not count, and that matters
The exclusion in the description is the whole trap. S.P.A.R.E. Crates are the cheapest thing you can make and the obvious way to burn scrap in bulk, and they contribute nothing at all to this achievement.
If your count is not moving while you craft, check what you are making. Crates are the one blueprint deliberately left out.
Pick something cheap and repeatable instead
Any other blueprint counts, so the efficient approach is to find the cheapest non-crate item you have unlocked and make it over and over. Scrap is the only real limit.
That means the two blueprint achievements feed this one directly: the more blueprints you have unlocked through Junkyard Architect, the more likely one of them is cheap enough to spam.
Scrap is the actual currency
Everything here runs on scrap, and scrap comes from the island: mobs, chests, drill rigs, and the piles lying around. There is no shortcut and no vendor.
Two hundred and fifty crafts is a lot of scrap, so gather it as you go rather than making a separate farming trip. Anyone already visiting daily for Junkyard Machinist and its hundred construction projects will accumulate more than enough along the way.