Well lots of people have shared their experience of how to safely replace the firmware of a Lite with flashme, but for those still hovering on the edge (like I was) here are some additional tips, not all of which have been well covered.
But first the critical issue, the tool you will use to short the SL1 bridge. In my preparation I foolishly ignored the advice of others on the forum. I thought it would be better to use a hard metal contact, a miniature screwdriver or a piece of insulated thick, copper, single-cored wire - both with only the tip exposed, the rest insulated.
WRONG, WRONG, WRONG! After 15 solid minutes of gentle jiggling I could not get past 1%. As my panic rose I hastily made what others have recommended. Using a wooden skewer (I didn't have a toothpick), I wrapped the tip 'sort of tight' with foil (very fiddly...I used about 1 sq inch of foil to start with) and then secured it in place by tightly wrapping cellotape over the foil only leaving a few millimetres of foil exposed.
This worked IMMEDIATELY and thats because the foil is compressible, so as you insert it and gently push down it shorts SL1 whereas a hard object seems to have major trouble making contact across the gap. Ok some people have got screwdrivers to work but my advice is HAVE SOME OPTIONS in case your single tool simply refuses to work! and I now agree the toothpick is the ideal tool
Some other tips.
You need to be able to carefully control the toothpick but at the same time you want to be able to see the DS's screen, so you can see if you are making progress. You can't do both without a mirror (or someone to help you).
So I sat in a chair at a solid table with good light, with my Lite plugged in to AC power (do not do this just relying on your battery!!). Have a small mirror (pinch one from your partner/mother/sister/daughter) to hand. With Passme2 and your flashcart inserted (loaded with flashme.nds) turn on your Lite.
Boot/run the flashme.nds file (however you need to do this with your gear). Press the required buttons on the DS until you have %0 displayed on the bottom screen (the % sign will be blinking). Being very careful not to accidently touch your on-off switch or disconnect your power lead, turn the Lite over and place it screen down near the edge of the table.
With a small screwdriver open and remove the battery cover. Place the mirror, facing you, on your knees, so you can see the edge of the table reflected in it. Gently slide the Lite toward you so that the bottom screen is off the table and therefore displayed in the mirror (with one hand supporting the Lite a bit and the top screen still on the table).
With your other hand you can now insert the toothpick carefully while at the same time seeing the bottom screen of the Lite reflected in the mirror.
It's a big relief when it gets to 100%....