Accessory belt idler pulley; easy to replace? Torque specs? 1995 Volvo 960

What is your question?

My idler pulley lost its bearing [empty hole in pulley hub], and the belt went limp. The replacement part ordered by the mechanic doesn't fit. I ordered one from the dealer. Can I put it on myself? I know how to release the tension on the belt tensioner. I think the idler pulley just has a central bolt. What should it be torqued to? Is this job as easy as it appears? Are there pitfalls to avoid with this DIY job? My serpentine belt looks good as I changed it 20,000 miles ago.