Since it's an old DS, you don't have the risk of accidentally shorting metal on the side, in the SL1 hole, like you have with the DS Lite, so all you really need is a mini screwdriver (flathead) that will fit in the slot and has a head long enough to bridge the two half-moon contacts, and you just have to make sure the battery doesn't fall out.
I think on an old DS, you can just keep the charger plugged in and you will keep power even if the battery falls out (but don't test that while flashing, duh! Test it before the flash), but the DS Lite does not have that ability.
Yes you have to remove that pad covering the SL1 hole and short the contacts--there is no other way.
Please read the several guides on this site and sticked on the forums regarding flashing. If you REALLY want to be safe, go ahead and insulate your flashing tool (except for the very tip, of course!) although I don't think that is necessary on an old DS, as I mentioned above. Still never hurts to do it for peace of mind.
Reading the guides, watching a flashing video, etc, will do a lot more for you than trying to get answers about "how to flash" here--trust me.