These are the things you will need:
Picture matting(the number of mats you will need will depend on the depth of whatever you are framing)
Tray(or pie plate) Mark the inner most part of the heavy cardboard,which will tell you where you want to spread the glue.You don't want to go all the way to the outer edge.If you do,the mat won't stick to the cardboard.Using the sponge brush,Paint inside the lines with a heavy coat of the white glue.
Press the cardboard into the sand.You will need to really press the sand into the glue.
You may have to repeat this process,to get a good coating of sand.But be sure to let each layer dry thoroughly, first.Or else you will end up with gluey,sandy mess.I know-I did that the first time I tried this.
While the sand is still wet,lay the sand dollar on it(you may want to glue the back of the sand dollar first),and gently press it into the sand.I learned on that one,too.
Let it dry thoroughly.Then give it a coat or two of matte spray urethane.This will keep the sand from working it's way loose,after it is framed.Learned on that,too.Sand drifting down onto the matting doesn't look very good.
(This one doesn't look good,but,then it's just for demonstration purposes.)
This is a miniature whelk I did.I also put footprints in the sand,with the foot of a doll.
You could use the sand from whatever beach you collected the shell,sea life,etc. on.I use craft sand,because it is finer.It just seems to work better.
The number of mats you will need will depend on the thickness of the sea life.
You can do this with a shell.If you do that,you will need to use a shadow-box frame,or a frame that has some depth. Or a ton of mattes!
My recommendation: stay with the shadow box frame.