We built this over the course of a couple of weekends. I think the key thing was making sure my 1x6's were the correct width before we began to cut all the other boards. There was a lot of trimming after the fact making the overall width 1/2" shorter than the plans. Some adjustments made to the original plans were...
1) Using decking brackets to support the plywood to the 1x8s on the trundle (super easy way to secure it)
This makes the bed easy to disassemble because although I swear I'll never move again I'm not building the bed into the room.
3) Instead of casters on the bottom we used those plastic furniture sliders. They were way cheaper and so slim. With the weight of the trundle wood and mattress they move flawlessly.
4) Since my daughter has a memory foam bed I was worried about sagging. We used small 1x? ripped in half along the inside of the side rails for a little lip and then 4 1x6's the width of the mattress to make supports and then laid plywood on top of that. 5) Cut down the headboard by 6.5" so that the mattress butted up against it.
Me personally I hate when there is a gap there for pillows to fall down in the middle of the night.