Drawings first. Isometrics for pipe, structural drawings or a 3D model for steel, and P&IDs plus general arrangement drawings for skids. If all you have is a sketch and a photo, send that; we can work from it, but the quote will carry allowances that a drawing would remove.
Material specifications matter as much as geometry. Carbon steel versus stainless versus chrome-moly changes the filler, the process, the preheat, and the price. Include pipe schedule, plate thickness, and any grade requirements, and tell us whether you are supplying material or we are.
Tell us what inspection the job requires. Visual only, or radiography, ultrasonic, dye penetrant, or magnetic particle on a percentage of welds? Do you need material test reports and a weld map delivered with the work? Documentation is not expensive when it is planned from the start, and it is painful to reconstruct afterward.
Finally, describe the site and the schedule. Shop fabrication with delivery is priced differently from field welding on an operating plant with hot-work permits and night shifts. A real required date lets us tell you honestly whether it fits.
With those four things in hand, most quotes go back within a business day, itemized so you can see material, hours, consumables, inspection, and delivery separately. That is a number you can plan around, and one you can hold us to.