Hi, Betsy. Here's how I would do it. First find the width between the outside edges of the horizontal beams. Then add 2 feet. Cut all the joists to this dimension and make an angled cut to partially clip the end as John2000 suggested.
Now stack all the joists together with the angles up and clamp them together with all the edges matching. Measure in 1 foot from each end and confirm that this is still the same measurement as the outsides of the beams. Mark this dimension across the top of the stack with a large speed square or framing square. Now measure in the width of your beam on both sides and mark this across the stack. Set your Skil saw to cut 2" deep and make a cut on the inside of both lines on an end. Now make a series of cuts about every 1/4" to make the area where the beams will go look like the teeth of a comb. Use a hammer and chisel to break out the "teeth" and square up the bottom of the cut.
Your joists should fit over the beam and making the cut for the slot is much safer this way.