What is the best way of joining multiple pieces of wood together?

If I ever do stuff like this, I glue them in place and then nail them as well.
I’ve made a box and marking jigs this way, they all seem to work fine.

Can you nail them to a backing?

I could nail them to a backing if I absolutely had to, but I’d prefer not.

On the other hand, If I have a backing, it can be something that she can hang up, which would be a plus.

Mending brackets, glue & screws would be my top choices.

There are special glues for this sort of thing. It would be good if you had some sort of clamp, or a way to hold the parts together for 24 hours while the glue sets. I agree wth the idea of a backing to help make it stable. I would nail it for reinforcement.

Biscuits are a good way to join pieces of wood.

It seems from your description that the joining will be along the long edges. And the best way to do that (normally, under most ‘construction’ conditions) would be to glue them along those edges and clamp until the glue dries.

But that would also assume some kind of finished edge, so that the edges would all be smooth and evenly planed to ensure no gaps between the slats. I doubt that you’re going to find pallet slats that are finished to that degree, or which will be thick enough (or flat enough, too) to take sufficient glue to make a good joint without gaps, and then be able to clamp together. (And I’ll bet that you don’t have enough clamps of the right size, anyway, or you wouldn’t have even asked.)

So my suggestion for you “in the real world” will be to obtain a piece of 1/8” or thicker luan plywood and fasten the slats to that by gluing and tacking (from the plywood into the thicker slats, so the heads of the tacks won’t show, and so that the tacks can “hold” in the thicker wood). That way you won’t have to worry about a complex (and fragile!) joinery project, when you’re not making a structural piece such as a door or table top.

IMHO, glue is absolutely necessary for long-term stability.

Only two weeks ago I made a small shelf out of some cargo pallet slats. I hand cut the wood and glued it together along the edges, staggering the ends, to make what looked like one ~8inch wide plank (the width of the slats varied a little) Think something like this:

11111111111222222222
3333333333333444444444

where each number represents another piece of wood. 1 was glued to 2 and 3, 3 was glued to 2 and 4, 4 was glued to 3, etc. I took some small pieces of scrap and screwed them to each board in my new wider plank to fasten them together while the glue dried. Getting overcomplicated I know, but I screwed 1 to 2, 2 to 4, and 1 to 3. This held them together snugly and it was easy to unscrew when done.

After the glue dried, I cut the new wider planks to the lengths I wanted and glued and nailed them into the shelf frame.

I haven’t quite finished yet. I need to sand down the front and back face to make them more even, and it takes some sanding to make cargo pallets look good anyway. But the wood held up fine with wood glue only to make the initial planks. I only ended up using nails for the frame because it was easy to do whilst the glue dried. Eventually I’ll need to seal the cracks and holes for the screws, but I plan on painting it. I wanted something that looked a tad buckly and uneven to match the trim in my bedroom, which is probably 70 years old.

Wood glue or crazy glue works the best for me and my family whenever we need it, and heavy duty tape or packing tape helps too

lay all the pieces together vertically then screw a baton horizontal across them at the top and bottom and sides this will make a nice frame

I have a friend who does this and then turns the most amazing bowls from them. Glue and clamp is all he does and he has a 0 failure rate. He doesn’t even plane or smooth the parts he glues, which I thought would be a necessity, he says the lumps and bumps makes for better joining. The most important part is to use plenty of wood glue, clamp TIGHTLY and don’t touch it for a couple of days to make sure it is COMPLETELY dry.

I can’t quite picture what you are doing, post a pic when you are finished, would love to see it. I just picked up a basket full of small pieces of cyprus, lopped off fence palings. The grain is beautiful, I am thinking about gluing them and carving a cat, the grain would make a great tabby cat!

For your purposes, I agree with @Elm1969. Use two parallel pieces of wood as ‘braces’ behind the slats (running in the opposite direction of the slats), and nail the slats to those. This will also give the piece something to rest on a nail for hanging.

Here’s a tutorial that shows what I’m talking about. Scroll down to see a pic of the back side of the wood ‘canvas’.

If this involves paint, be sure to sand away any glue that squeezes out of the joint or you will have a challenge to paint it.