I did most of the work on this at night, so the pictures aren't so great.
Thanks doing it first, now it's on instructables:
Step 1: Materials & Tools
1) 6 8' long 2x4's
2) 2 5 gallon buckets (I used these: http://www.lowes.com/pd_4853-1152-57640_0__?productId=3083565&Ntt=5+gallon+bucket&pl=1¤tURL=%3FNtt%3D5%2Bgallon%2Bbucket&facetInfo=)
3) 2 80lb bags of quickcrete (I used these: http://www.lowes.com/pd_10385-286-110180_0__?productId=3006075&Ntt=quickcrete&pl=1¤tURL=%3FNtt%3Dquickcrete&facetInfo=)
1) saw (I used a circular saw, but table, reciprocating, or hand saw can all get it done)
3) broom handle or something for mixing quikcrete
4) something to measure quarts or liters
Step 2: Measure and Cut and Connect the 2x4s
I cut 2 of the 2x4s into 5' and 3' pieces
I cut the next 2 into 5'2" and 2'10" pieces
I cut the last 2 down to 5'6" and discarded the rest
DOUBLE CHECK THESE MEASUREMENTS
The height of the 5' section should be right around armpit height, the 2'10" section should be a comfortable height to put the bar down on while benching, and the additional 5'2" and 3' sections just need to be tall enough that you're confident the bar can't role off of the 5' or 2'10" section.
Using 2 1/2" deck screws, screw them together in pairs keeping the screws about a foot apart, and making sure to stagger them so that screws never hit each another.
Step 3: Mix the Quikcrete
Here's how I did it: about 3/4 of a bag of quikcrete plus 3.5 liters of water. it will mix into a much more liquid-y mixture and will not cure as strong as the right way, but you won't need to bother with mixing in another container and pouring a thick mixture around the 2x4s. Doing it my way the concrete should be strong enough and you can mix each in a bucket and simply sink the wood into it.
Sink the posts into the quikcrete, and use a level to make sure they're plumb. Use scrap wood, saw horses, etc to hold the posts in place. Give the quikcrete 2 days or so to set.