Welcome to the forum! I love the designs you've chosen.
I'd say my best tips are:
1. Get organised. Make sure all the colours you'll need are set out in numerical order so you can find them quickly and easily
2. Use a fabric calculator and make sure you have enough fabric, because it would be terrible to put in pages and pages of work, only to discover you're short at the bottom.
3. Hem your fabric or use fraycheck or something similar. You'll be working on this project for a long time, and you don't want to risk your fabric fraying too much.
4. GRID YOUR FABRIC! Even if you don't usually grid for projects, I'd highly reccommend it for HAED. Lots of colours, many very similar, are a lot easier to work with if you grid.
5. Used a frame or hoop of some kind. If you're a stitcher who usually just holds the fabric, you may find that the sheer size of these projects makes that far more difficult.
6. Make sure you've experimented with different numbers of strands on different fabrics till you find the coverage you like best. (I', working with 2 strands on 18-count for my storykeep, but I think I'd only use 1 strand if I were using a higher count fabric, but that's just me - some people use 2 strands on 25-count too).