I like nice big juicy seeded watermelons. You could choose an unseeded one, but seeded watermelons are so much sweeter and I think you'd probably want to remove the white seeds anyway because they probably wouldn't puree very well.
To remove the seeds, I cut very thin slices and used a fork to poke them out.
I removed the rind and pureed the meat of the melon. I used a wand, but you could use a blender or a food processor.
You could use popcicle forms or small paper cups, but the grandkids that frequent here most are pretty little and probably wouldn't finish anything much bigger so I poured it into ice cube trays and froze it. (In some of these pictures, the melon looks kinda orange, but I assure you, it was a beautiful red!)
Once frozen, pop them out and put into baggies!
It's a little bit of work, but the smiles I get paid for it will be well worth it!
God Bless You All!