To determine the index Miller, you need to know the crystal systems and lattice constants. If you know all this, you can use one of the standard programs indexing or do it manually using the extinction laws for this symmetry.

You can use  avalaible free software:

DICVOL06 for indexing:

http://www.ccp14.ac.uk/solution/indexing/

http://www.ccp14.ac.uk/ccp/web-mirrors/dicvol/

http://www.ccp14.ac.uk/ccp/ccp14/ccp14-by-program/dicvol/

and Power Cell for simulation:

http://www.ccp14.ac.uk/ccp/web-mirrors/powdcell/a_v/v_1/powder/e_cell.html

Best regards,

Pedro

Yes...you need to use Braggs equation first. Then you will have a set of lattice plane distances which describe the length of vectors in reciprocal space. The challange is to find a 3D arragement of points which fits to all of these lengths. Typically, the best way is to start with the longest vectors and make ratios. If there are some relationships like sqrt(2) or sqrt(3)  you perhaps find the plane diagonal or cube diagonal of the using cell. integer factors tells you that you have a second or third order of another hkl. It needs a bit time to find a solution so that automatic procedures are doing this in software packages. Especially, for non-cubic phases this is not trivial. But the principle is always the same. You need to find lattice parameters which enable an indexing of all interferences by integer numbers of hkl.

Follow P.G. Gallardos advice and download a software. However, you should perhaps read a little bit more in order to understand what theses softwares are doing. Using a black box is ALWAYS not helpful, especially no software is free of bugs...and this does not mean that a software cryshes. It simply give a wrong number since "+" has been mistyped by "*" but no visible error message appears.

Thanks a lot all of you