It’s a concrete base I assume ?
get that level, the rest is just timber framework, cladding and doors, roof trusses (I done a cut roof on mine with overhang along the front), board the roof out then roof sheets,.
tape measure, chopsaw, screwdriver and nail gun, it honestly isn’t as difficult as everyone has you believe. 👍
If it makes it any easier, break it down into square panels, mine are 12ft x 7ft panels.
cost wise, you won’t save a fortune over buying them, but what you will do is build a much better spec out of better materials, mine is all 4x2 and the usual bought ones are 3x2, twice as many roof rafters @6” as opposed to the normal 4”, doors and vents where they worked for me, and as mine is going to be for poultry I done built in nest boxes, feeders, water tanks etc, so a full custome build that would have cost me an arm and leg off the shelf.