You've probably needed to glue two pieces of wood together or repair a stripped screw hole in a door jamb using glue and wood match sticks. If you reached for a bottle of yellow glue, you did the right thing.
Carpenter's glue is far more robust than the white glue you see children use in kindergarten to create artwork using colored construction paper. White glue is great for paper and cardboard, but it just doesn't have the claws to hold wood together considering the abuse wood's expected to endure.
Over the past few years, yellow carpenter's glue has gotten better with advancements in glue chemistry. Not only is it affordable, but you can also easily find one that can withstand exterior use, allowing you to glue together wood that can resist getting wet.
DEGREE OF DIFFICULTY: Two hammers out of five
Step one: Stop and think what job you're trying to accomplish. Will the wood be always indoors, will it be outdoors or will the wood you're gluing have the occasional exposure to water? You need to read the label of what you're buying to make sure the glue is suitable for what you're doing.
Step two: When you read the labels on the glue bottles, don't confuse water resistant with waterproof. Water resistant means that the glue should hold the wood together if it gets wet occasionally or if the exposure to water is minor and the wood dries fast. Waterproof usually means that the wood pieces you glue will stay together if the wood stays wet, but sometimes the instructions will warn about continuous submersion.
Step three: Just like any glue or adhesive, the surfaces you're gluing need to be clean and dry. Common sense should tell you that dust and dirt will interfere with the ability of the glue to get in contact with the wood. As for the water, that's a subtle requirement you might not understand until you think about it.
The reason the glue holds wood together is because on a microscopic level the glue soaks into the wood fibers. Once it does this and dries, the glue interlocks with the wood fibers, holding the two pieces of wood together. It's similar to a bur getting stuck in your sock or on your pants. If water molecules are filling up the space where the glue wants to go, the glue can't interlock with the wood fibers.
Step four: Realize the common yellow glues at hardware stores and home centers are generally not suitable for structural applications. Major lumber mills that make laminated beams and other engineered lumber use special glues that are formulated for high-tension structural jobs. Once again, get the right product for what you're attempting to do.
Step five: You get the best results when the pieces of wood being glued together fit tightly. Don't expect the glue to be filler between gaps and have great holding power. That's not how it works. Shape, sand and plane the wood until the pieces fit with no visible gap.
Step six: Apply a generous amount of glue to the two pieces of wood. You can use a small brush and apply the glue as you might paint. The glue should have the consistency of fresh latex paint.
Step seven: High-quality yellow carpenter's glue has a strong initial tack. This means the glue will grab pretty tightly right away. You'll have a few moments to adjust your pieces of wood, but probably not more than 30 seconds. If the wood is highly porous, the adjustment time is just a few seconds, so practice with scrap wood if you're not sure.
Step eight: To get professional results, you need to clamp the pieces of wood together. Squeeze clamps work great for this task. Don't hope you know how to use them. Practice first with the wood pieces before you have glue on them. Once the glue is applied, you should not be fidgeting with the clamps.
Step nine: Excess glue will almost always squeeze out between the pieces of wood. If you intend to stain the wood, don't wipe this glue off. Let it harden and sand it off. If you wipe it with a sponge, the glue will fill the pores of the wood, blocking stain from penetrating the wood fibers. If you're going to paint the wood, use a damp sponge to remove the excess glue.
Summary: In all honesty, it's cave-man simple to use yellow carpenter's glue. As with just about anything, if you've never done it, practice on some scrap wood before you attempt to do the real project. Don't purchase cheap glue. The more expensive glue almost always contains better ingredients and will perform better over time.