I am frequently asked about my process of mounting paper onto wood or museum board. I've spent YEARS refining this process so I apologize in advance for the length of the explanation!
REASON: I mount paper on wood or board is because I prefer to transfer the delicate details of my sketches via light box rather than using transfer paper or similar methods. To use a light box your surface has to be thin enough to allow light to pass through (paper), but to paint without having your surface buckle or curl you need to work on something sturdy enough to hold up against water and the test of time (wood or board). My process combines the best aspects of these two surfaces.
DRAWBACKS/PRECAUTIONS: Though paper mounted on wood/board is more structurally sound than unmounted paper, if exposed to the elements (moisture, sun, dirt...etc) it will still deteriorate. If the work is commercial then this is not a particularly big worry, but if the work is for a gallery then I recommend framing the piece behind UV-blocking Plexiglas (for gouache/watercolor/graphite/oil/acrylic) or sealing the piece when it's done with an archival non-yellowing spray or varnish (for oil and acrylic.) Not absolutely necessary... just something to consider. One other thing; oil paints in particular are corrosive to paper and can yellow or literally "eat away" at paper over time. Some people consider paper an unsuitable surface for oils for this reason. I love painting oil on paper, but I always seal the paper with a combination of sprays, clear gesso, matte medium, and acrylic-paint washes after transferring and mounting the drawing but before painting. Again... food for thought.
PAPER: I recommend using a heavy printmaking paper such as Stonehenge (favorite) also suitable are Somerset or heavy Rives BFK. The process will be really hard or impossible if you use thin paper as it tends to buckle and tear in the mounting process. If your light box isn't very powerful be sure to get white paper as opposed to grey or cream.
MOUNT: I recommend archival wood for gallery work and museum board (~1/8") for commercial work. Museum board is expensive but won't buckle as much as illustration board. Be sure that if you are mounting on wood, hardboard, or gesso board that they are archival (AKA: non-acidic/won't leach chemicals) or they will yellow your painting and may even eat away at the paper itself over time.
SKETCH: I recommend you b/w "Kinko's Copy" your sketch to increase the contrast, darken the lines, and adjust the size of your sketch to make transferring easier.
"GLUE": You MUST use Matte Medium. There is NO substitute. I've tried so many things, just trust me on this.
SUMMARY OF IDEAL MATERIALS:
*Archival wood or museum board
*Matte medium (no substitute)
*Fine-grain sandpaper wrapped around a flat 2"x5"x1" thick piece of wood (or something similar)
1) I trim the paper 1/4" bigger in length and height than the wood. (So the border is 1/8" all around when mounted)
2) Tape the sketch to the back of the paper using artists tape so it won't shift around and transfer your drawing on a light table. (You can tape it to a window in the daytime if you don't have a light table.)
3) Choose a location to mount; it should be flat, level, clean... any grit or dings in your surface will be impressed into the surface of the paper as it is being mounted. Put down an even layer of paper towels that extends past the size of the paper in all directions (to absorb moisture and cushion the paper.) Place your paper on the paper towels, face/drawing side down.
4) Put one additional piece of paper towel to the side (for later) and gather your matte medium, gesso brush (the bigger the better), and the wood or board you plan on using.
5) Squirt a generous amount of matte medium onto the wood/board and immediately start spreading in around with the brush, sweeping the excess off onto the spare paper towel. You need to work FAST but still achieve an even coat. Use the brush to glob just a little extra Medium onto the corners and along the edges if the size of the piece is large. This will ensure that those key areas bond well. This part should take no longer than 1-3 minutes depending on the size. It's best to have a friend help if you're working larger than 2'x2' to be sure that you work fast enough that the Medium doesn't dry before you can mount.
6) Flip the wood/board over, holding it above the back of the paper/drawing. Visually line them up and lower the wood/board onto the paper. Press down gently to force the Medium to touch the paper, being very careful not to let the wood/board shift around.
7) Flip the whole thing over, including the paper towels. Smooth the paper down, using circular motions and working from the center out to push out any air bubbles and to flatten out the Medium. (Keep the paper towels flat between your hands and the paper/drawing to keep the paper clean and to prevent the friction from compromising the integrity of the paper's surface. Use your hands to bend the excess paper just a bit around the edge of the wood, this further ensures that the edges bond strongly.
8) Flip the whole thing BACK over again so the wood is facing up (and the paper towels are flat on the bottom) and put a piece of paper or paper towels on the back as well and pile heavy books on top. Make sure the weight covers the whole back and is even, the paper towels on the top will protect your books from any matte medium that seeps out.
9) Leave it for 2 hours at a minimum. Leave overnight if possible, the larger the piece and the more matte medium you use the longer it will take to dry. If you only have time to leave it 2 hours, use a hairdryer to be sure it's dry and fully set. If you don't fully dry it and the paper is even imperceptibly moist from the matte medium the surface will be more prone to damage as you start painting/working on it.
**If you are mounting on board, just trim off the extra paper with a sharp Xacto knife and you are done! (Cut from the front, using a cork-backed ruler and trimming a little of the board too for a cleaner edge.)
**If you are mounting on wood, continue:
10) Remove books and VERY GENTLY test a corner by tweaking the excess paper to check that it has properly bonded to the corner of the wood. Place paper down/wood up on a cutting board and trim the excess paper with a fresh Xacto knife. Do not trim too close to the wood, maybe leave ~1/32" still sticking out.
11) Flip back over so the sketch is up and take your fine-grained sandpaper (wrapped around something flat so you sand evenly) and use firm downward strokes at a 90° angle to the edge of the wood to sand off the remaining paper and to further strengthen the bond at the edge. This will also make it so that nothing can "catch" on the paper and rip it off the wood in the future. (Warning! Downward strokes ONLY! Upward strokes will catch on the paper and pry it away from the wood.)
12) DONE! I've tried to be as specific as possible, but the only way to perfect the process yourself is to practice.
I hope this was helpful to someone out there! Don't be intimidated, it probably takes longer to read all this shit than to actually mount the paper.