How to add a roundabout to a pre-existing junction?

Following a practice I have found this to be the easiest procedure.

1. Zoom right in to max at edit point
2. click edit choose Potlach2 and click the plus + to mag a bit more
3. Click on road where it will join mini roundabout while pressing shift to create a node and click scissors symbol to cut it
4. repeat for other 3 roads
5. click on middle node and with just that highlighted click bin/delete symbol.
6. highlight and delete to other bits
7. click and highlight one of the four roundabout nodes now create a new node between this and the next of the four and continue until you have rough circle of 8
8. use circle tool to make neat
9. While still highlighted tag from box on left as a suitable road type and choose the correct one way tag
10. if all looks good save it and write for example "Mapped roundabout using bing, survey and local knowledge". that's it, job done
11. It would take five minutes with practice.

I've taken Andy Mackey's answer, but just switched around a few of his points to make it a lot easier (for me at least). Where he starts, making nodes on the original road(s), made them behave (sometimes) erratically (i.e. deleting a complete road).

1. Zoom right in to max at edit point.
2. Click edit and zoom to a decent detail-level.
3. Make a circle (your future roundabout) and click on the 'circle' tool to make it round. MAKE SURE that the roads that will be included in the roundabout, will have a node ON them. Do this by clicking ON the road(s) while making your circle (you should see a node point with an extra circle around it).
4. With your circle still highlighted, add the appropriate road type (go to the left window), be sure to also pick the 'Roundabout=YES' type from the most-right tab of this window. This will automatically set it as 'one way'. Make sure the one-way direction is correct, if not, click on the 'change-direction' icon.
5. Click on one of the roads to highlight it, then click on the node/intersection with your future roundabout.
6. Use the 'scissor'-tool to divide it.
7. Do this for the other roads also.
8. Click on the roads INSIDE your roundabout, and then use the 'Bin' to delete them.
9. If all looks good, save it and write for example "Mapped roundabout using .........".

Good luck !!!

You could read this http://wiki.openstreetmap.org/wiki/Tag:junction%3Droundabout If the area as got good bing it will be easier. If not I would sketch a plan, walk around the site and get some traces. If you mess up just don't save it. You could find a blank area on the map to do a practice, Do not save this practice of course. edit... I have now had a good look at this in Potlach2 you need to plot a few nodes around the mini roundabout you see in Bing, use the circle tool in the box lower right to make the circle neat.Now make it one way anti clockwise as it's USA. next connect the four roads to it by clicking their meeting points and pressing "J" for each of the four. Then remove the central node, make sure that's the only high lighted node when using the bin/delete symbol.Update the undo and redo boxes are useful,and after practice the problem with this method is that the circle tool sometimes moves the centre a bit upsetting the alignment. so use my other answer for best results.