As noted, the wheels may come assembled. Also, sometimes I get them in bags of ten. In that case, I re-use the bags; even though the label on the bag says ten, you should have 12.
In case you do need to do the assembly: Push one bearing to the shell of the idler wheel. Using a bolt as an alignment guide, put a precision washer between two bearings and push a the second bearing to the shell as well. Remove bolt. Do this to all idler wheels; you will end up with twelve of these:
- 625-2RS bearing x 2
- precision shim washer
- idler wheel shell
Do not mix precision washers with regular M5 washers!