Gluing up smaller boards to make a larger panel, like most tasks in woodworking, seems easier than it really is.
Lets say you need a panel that is 10" wide (the length does not matter at this point), and you have a 4 inch wide board to use to make this panel. You'll want to cut the 4" board so that you have three sections to edge glue together. This will give a slightly larger board than needed so that you can have enough scrap to square the panel after the glue up.
If possible, do not plane the boards to final thickness. If your planer is wide enough to accommodate the final board width, this is ideal. During the glue up, the boards may not align perfectly, and a board may glue up higher (or lower depending on which side you look at it).
If one board glues up 1/16th higher, in this case you are loosing double the thickness of the height difference to make the panel flat again, 1/16th on both sides. If you were at final thickness before glue up, you must be extra careful for this.
Here's the first tricky part. You'll want the best looking panel possible, so you will have to rotate and flip the boards to get the best grain match you can. But at the same time, you will have to pay attention to the grain direction on the end of the board.
You want to avoid having the growth rings turning all in the same way. Otherwise you may have trouble with the entire board cupping in that direction over time due to relative humidity changes. Alternate the grain direction as much as possible to avoid warping in the future.
Some woodworkers say that this is not necessary anymore due to our air conditioned and heated houses, but it doesn't hurt the process as long as you can get the desired final appearance of the panel.
After you've decided which boards you will use in what order and what faces are the good faces, the next step is to mark them somehow so that you can replace them the same way after you have machined them. The standard way that most woodworkers do this is to draw, with pencil, a triangle over all the boards. Not only does this method tell you the sequence and face of the board, it also indicates the position of the board relative to one another.
It is not necessary to use biscuits or any other joinery technique when gluing up wood panels edge to edge. This is one of the strongest wood-glue bonds. Using biscuits will only make extra work and introduce the possibility of more errors.
Once you have your boards ready, try to get the glue up done that same day. Leaving the boards sit for a long period of time after you have edged planed the boards, and you will notice that your once perfect alignment is now out of whack due to humidity changes in the wood causing the boards to ever so slightly warp and twist.
When you are ready for glue up, get all of your clamps and cauls prepared on a flat surface. Do a dry run to make sure that everything will go smoothly.
It is a good idea to mark your boards in some way so that you know which edges will be glued to which edges. As you turn each board to apply glue, it is easy to flip one the wrong way down. To prevent this, draw a shape like a triangle across the boards so that you will be able to align them exactly back the way they were when you had dry fit them.
This example has only two boards. With three boards for example, one triangle can be drawn across all three, or two triangles across the two joints (one higher up, and one lower down so that they do not line up at all, to avoid confusion).
Within your clamps, stand the boards on edge in their proper order. Glue up the boards by running a bead of glue (usually PVA glue is best for this application) over one edge of each board. I usually smooth it out with a small sponge roller to make sure the entire edge is covered. (Or you can use an easy to Make Wood Glue Dispenser.)
Then, one at a time, lay them flat and start to align them properly.
You will notice that the glue acts like a lubricant and that the boards will have a tendency to slip out of place. One way to avoid this is if you own a pin brad nailer gun. Put two pins into one edge of each board. Another way if you do not have pin nailer is to use a staple gun and cut off the flat part so that you have just the two tines of the staple slightly sticking out. This will keep the boards from slipping.
I would only really recommend this method on larger panels with many boards as a safety precaution. Otherwise, a small glue up is probably manageable without this alignment help. Again, the markings you had made with the triangle will help you notice any boards that slip out of alignment.
The last step is to clamp your panel together. Using your caul, clamp the boards so that a line of glue squeezes out of each joint. You don't want to over do the pressure as it will cause too much glue to squeeze out. This will create a glue starved joint which will be prone to failure. (Plus, your entire board may cup under the pressure and the clamps will tend to lift the two outer boards.)
It's hard to say exactly how much glue should come out because this will depend on the amount of glue that you applied, but a thin line or connected beads of glue should be visible. If there are long areas of connection points along the joint that do not have any excess glue coming out, then not enough glue was applied.
After about 20 minutes (with PVA Glue, Polyvinyl Acetate) you can remove the clamps from the boards.
At some point near this time frame, you can also take a Sharp Cabinet Card Scraper and scrape the glue off the joint if it is reasonably hardened, something like a rubber consistency throughout. If you try to remove the glue too early, it will smear across the face and be a big pain in sanding and finishing if you don't get it all off. Wait too long and the glue will be totally hardened and will be mush more difficult to scrape off.
Let the panel sit overnight on a flat surface before working with it.