Hi vdao

This could turn into a rather long answer, but I will try to make it as short as possible. First you need to know the requirements of your referencing code. (Is it B31.3, ASME VIII etc?) These different codes will have different requirements for the impact test temperatures, and the resulting thickness ranges qualified. Also, do you have any additional client specification requirements? Broadly speaking, the following will be appropriate:

1) For impact testing, the Group number is a suplementary essential variable, so make sure your coupon material is the correct Gr no also.
2) Test piece is similar than for non impact testing, except you need it to be somewhat longer to get the impact tests out. (Around 500mm should be OK.)
3) Vertical welding progression is also a suplementary essential variable, so if you need to weld out of position in production, make sure you are welding vertically up on the test piece as well. (e.g. plate in 3G, or pipe in 5G position)
4) Generally the code calls for impact testing in the weld metal and HAZ. If you are welding two different materials together, (e.g. a P1 Gr 1 to a P1 Gr 2) then you will need to perform a set of impacts in each of the materials' HAZ's. Here is where you need to be very carefull of client specs, because some client specs call for a lot more testing. (Possibly: Weld metal; Fusion line; Fusion line + 2mm; Fusion line + 5mm)
5) The impact test temparature is usually the minimum design temperature of the component that needs to be produced, but make sure the the client spec does not call for something different. Typically P1 materials are only used confidently down to about -40°C, but lower is possible.
6) I am assuming the .25 means 1/4" material thickness. If this is the case, then you will probably have impact specimens of 5mm width. This means that you will have to have a reduced impact test temperature to compensate for the reduction in thickness. (The referencing code will give you this information.)
7) Typically the code calls for 27J average for a set of 3 specimens, with a minimum single value of 20J. This is for 10mm wide specimens, so 5mm wide specimens need half that. (For ASME codes these are typical values.) Again, just check your client specs, because these can be substantially higher in some client specs. (e.g. 45J)
8) In the WPS testing, NACE practically just means that you need to perform hardness testing, and that the maximum hardness needs to be < 250Hv. (For P1 materials) Again, check the client spec for any specific hardness testing requirements. Some client specs call for a lot more testing than the code asks for.
9) Also, the filler metal must contain less than 1% Ni for NACE compliance. This is an important consideration, because often a Ni bearing filler is used to improve the weld metal impact properties. In this case, get one that will deposit less than 1% Ni. (They are usually made to deposit around 0.9% Ni, specifically to meet this requirement.)
10) The impact test can often fail with FCAW if your heat input is too high, so keep the heat input low, especially for a thin material.
11) Usually a good idea to let the lab also perform macros on the hardness test pieces, as other codes require this, then you will have it available for the future.
12) The usual tensiles, root bends and face bends must also be done.

I have probably left things out, but that should get you on your way. - I think I need to go and rest now....

Regards
Niekie