1. Pause, observe the local lay of the land, identify anything unusual.
If rolling clouds of mysterious colour, raining blood, acid or disease, certain vermin are around, dig down into the soil immediately and upon setting up preliminary stockpiles, begin carving into stone.
2. Set up a forge and begin producing tools, weapons, crossbows and ammunition at once. Set up the first military squad and commence training so that the outpost has dedicated defence from the start.
3. If nothing immediately hazardous has been identified, begin setting up refuse stockpiles, stockpiles, graveyards and farms, preferably 30- tiles away from some sort of water source (if none is found, it is no worry at this point).
4. Chain up guard animals by the front entrance, build a barracks by the stockpiles.
5. Set up the carpentry, distillery and masonry works to prepare for the arrival of migrants.
6. Construct dedicated bedroom quarters for the migrants, once the first 2 waves have arrived begin preparing for the arrival of merchants.
7. Construct more specialized industry works like the kitchens, craftsdwarf workshop, farmer's workshop, mechanic's workshop and permanent forges e.t.c. (constructing the trade depot once cloth crafts begin getting pumped out)
8. Expand the military and industry to foster enough logistical support to a full time army and a sizeable militia. Attempt to acquire war animals of some sort to provide auxiliary sentry duty with expendable units.
9. Begin fortifying the surface and the caverns, begin placing traps to acquire more exotic animal species.
By this point the Fortress should be ready for its first ambushes, sieges and megabeasts.
10. Set up a hospital and the most advanced industries (including a transition to magma based forges if possible).
11. Begin employing the most advanced alloys or metals available, smelting down obsolete metals for trade.
12. Begin smelting down inferior quality crafts, aiming to get all weapons and armour to at least =exceptional= quality.
13. Begin employing the child workforce for agriculture, in harvesting food, booze and textile crops, in anticipation for their entry into the adult workforce.
By this point the Fortress should be ready to take on megaprojects with low casualties