Probably, it would be easiest to add the content of a Fleet bisacodyl enema to the enema water, but those are not available in Germany. Two other ways were described in the forum before: crushing Dulcolax tablets (with or without removing their coating), or melting the suppos and then adding the resulting powder or solution to the enema water. The drawback is that both the crushed tablets and the melting suppos float on top of the water, and thus will not flow where you want them to be. Constant stirring is not an option, at least not when you give to yourself without a helping hand.
I found another satisfactory way of doing it. Probably very few of you use those small and thin rectal nozzles that often come with the enema bag. My favorites are the flexible rubber nozzles, which have a front hole and two holes at the side, and come in different sizes. I found out that, except for the very small ones, the front hole is wide enough to slide a Dulcolax suppository in. The nozzle may cut off a few flakes from the suppository, but those can easily be moved into the nozzle with your fingers.
Hence I proceed as follows: I prepare my enema with water slightly on the hot side, maybe 39-40° C. This temperature will make it easier to melt the suppo. Afterwards, I push a Dulcolax suppository into the nozzle as described above. When taking the enema, the constant flow of the water will gradually melt the suppository and move it to all parts of your bowel. If the suppository blocks the front hole, the flow will simply be redirected to the side holes. Don't take the enema too fast, or the suppository will not melt completely. 2-3 minutes for 1 quart is a good timing. Also, I found out that 1 suppository per quart is a good amount (for me). So after the first quart, you either have to refill the bag and insert another suppository, or, if the bag contained 2 quarts already, stop the flow after the first quart and insert another suppository into the nozzle.
For me, this is the easiest and most efficient way to do it. Also, there will not be any residues of the suppository sticking to the bag.
Try it, and enjoy! I hope you will share your experiences (including holding time!) here.