If you simply specify a number of charges they will auotmatically be arranged across the field. See the discussion on arrangements for more information.
Charges may be placed in a specific location by prefixing or appending the charge descriptions with one of the following:
Central locations (from top to bottom) In Middle chief, in base, in honour point, in fess point, in nombril,in abyss
Side locations - in dexter/sinister chief, in dexter/sinister flank, in dexter/sinister base, in flank (the last puts a copy of the requested charge in each flank
In quarters - in first/second/third/fourth quarter
In shield halves - in dexter/sinister side/half, in upper/lower half/part. These are not strictly heraldic constructions but are a useful "fudge" until DrawShield does a better job with shields split into two distinct parts (such as those common in European Heraldry).
You can place charges on an ordinary. In most cases he space available to do this is quite restricted some only a small number of charges is allowed, an error message will be generated if there isn't room to place all of the charges. Similarly, some numbers of charges don't really make sense on some ordinaries, again an error message will be shown if this is the case.
You can also place charges in relation to an ordinary by using the contsruction ordinarybetweennumber charges, or number chargeswithinordinary. As usual this only makes sense for certain ordinaries with particular numbers of charges.
Particularly with the bordure it is also possible to use the construction within [an ordinary] [one or more charges].