The size and location of DG depends on the objective function, i.e to reduce losses, increase system stability, or system reliability. you must identify your objective function first.
1) Decide an objective function, like minimum power loss, minimum voltage drop in distributors, improved voltage profile, better voltage regulation at end nodes, etc.
2) First manually place the DG randomly on different buses and prepare the result summary for above objective functions.
3) Use any optimization tool like genetic algorithm to find out the most suited bus to place DG.