Before working on the final area, set a little scrap of the same paper, cardstock, fabric, or wood blank next to the work. Make one brush stroke, one glue dot, one paper cutout, and one little overlap there. One little test may show more than an extended plan as craft supplies may react differently as soon as they hit a real surface.
For example, with paint, if it feels right from the can, it may seem streaky on textured paper or too shiny on a sealed wood blank. A soft brush may create thick marks depending on how much it holds. Doing one test layer of paint on a little scrap shows if the surface needs to be prepped, what color it may take on once dried, and if an additional coat will make the piece more even or too thick.
Glue is the same. If it has too much glue, it will cause the paper to wrinkle, the fabric to stain, the beads to shift, and/or show a bump near the edges of the cutouts. Doing a small amount test on a scrap shows the correct glue quantity and how long to wait for it to set. You can see if the glue dries clear, glossy, cloudy, or bumpy. Small things like that are very important for handmade cards, decorations, panels, ornaments, and other pieces where the surface is important.
Paper tests are more than glue. Do a little cut of the paper and look at it. Some paper cuts softly, some cardstock shows a hard white cut line, and some painted paper curls after moisture. If you plan to fold, trim, stencil, or layer paper, then a little cut test shows whether you can make it work or not in terms of keeping it flat, if the border has to be bigger, if scissors are needed, or if something should be measured more precisely.
A scrap test should not have to be its own project. Keep it small and targeted and try the same color, same glue amount, same cut shape, or same texture layer as planned. Wait enough time to see how it dried, bend, tap, and rub lightly to check that things are holding where they should and if it feels rough or not. If it looks awful on scrap, your piece is not ruined yet; it is saved. You have found the problem while it is easy to change.
It also calms the decorative choices. Rather than wondering what will work with your fabric, beads, paper, paint, and varnish, have the materials show you first. That leaves more of the final piece with the actual work instead of testing. A little scrap by the table may not seem as if it matters. It may mean the difference between your paint getting smudged, paper getting wrinkled, glue showing, and things getting rushed.