Visa Run To Phnom Penh

I will need to use up another 3 months of my non-b visa and would like to go to Phnom penh for a few days in the process, can someone give me an idiots guide to arriving: whats needed at Cambodian immigration, and also any good recommendations on a room, I would like to stay in a room overlooking the river and am prepared to pay around £15/$30 per night, any ideas?

Thanks in advance