Most transparencie's contrast substantially exceeds Ilfochrome's reproduction capability. This can be corrected by dodging/burning or using a contrast mask. In general, the contrast mask is a better route as it lowers the contrast of the entire image to fit with the paper's contrast. Dodging/burning only addresses selective areas of the image.
The lower contrast Ilfochrome material is helpful but still only addresses about 40% of the images in my experience.
For the life of me, I have no idea why people allude to contrast masking as some superhuman effort requiring Jedi tricks and use of the Force to accomplish. Anyone, I repeat ANYONE with compentent black and white darkroom skills and a modicum of cleanliness can make a contrast mask.
Go to an art supply store, buy some 3M graphic arts tape - comes in the blue plaid box.
While your're at the art store, get some frosted Mylar or frosted acetate - whatever they have. Cut some 4x5 pieces when you get home.
Get a contact print frame. I prefer the hinged type with the spring back as I can leave the small side attached to the print frame and open/close the larger side as the working side of the print frame. Also, helps prevent the film from shifting that will (at some point) happen even if you're super careful if you slide the entire back into the print frame.
Get an Ilford anti-stat cloth (if they're still available). Also, get an anti-stat brush. Get canned air.
Check the print frame back for lint, dust, etc. Blow off with canned air. Make sure it's as clean as possible. Clean the glass on the print frame inside and out with a good quality glass cleaner using lintless paper towels. I like Kim Wipes.
Blow off the glass on the inside of the frame & outside. Brush the glass lightly with the anti-stat brush to ensure you've neutralized the stat charge from the air as much as possible.
Put the back on the print frame engage the spring clip on the small side - and use the hinge to open/close the larger working side.
Get your transparency. Clean it using an anti-stat film cleaner. Blow off any remaining particles using canned air. Using the Ilford cloth or film brush, give the transparency a final neutralizing brush or wipe. Check the both sides of the film for dust by holding it under a strong light at an angle.
When you've verified it as being clean put it into the print frame emulsion side down in the lower right corner about 1-inch from the bottom of the frame and 1-inch from the right hand side of the frame. Using a piece of the 3M tape, tape it to the glass on one side so it won't move. Put the piece of frosted Mylar over the top of the film. Close the print frame back.
Turn out the lights. Get your 4x5 film (more about that later) out of the box, and verify that it is emulsion side up. Open the print frame back, place the film in the lower right hand corner of the print frame and close the back. Lock the spring clip so the back won't flop open. The film will now be back to back. That's right, you're exposing through the back side of the film.
Flip the print frame over, give the glass a blast of canned air to ensure no boogers are on the glass, and expose the film.
Now for film and processing.
I like to use Ilford FP-4 and Ilford ID-11 for masking. I use 4x5 film for 35mm through 6x7 formats, and 5x7 film for 6x12 and 4x5 formats. The larger size makes it easy to align in the print frame, handle in processing and cut out to leave tabs for registration.
I use an Omega D2 enlarger with a Beseler/Minolta colorhead. You will have to do some trial and error exposure tests for your equipment - that's just part of the process.
Run the enlarger head to near the top of the column and set it at F/8 for starters. Try exposing for 5 seconds. Process in ID-11 (or D-76) diluted 1:3 for 5 minutes.
After fixing, look at the film. You should be trying to create a negative that looks like you'd have to print it with grade 5-6 black and white paper. You want density in the highlights that tapers off at the midtones and has NO density in the shadows.
If the density is too great - try a smaller f/stop, adjust the exposure time, etc. until you get the combination that gives you the density needed with the developer dilution and 5 minute processing time.
Registration: yep - you can use registration pin punches, if you can find one. Otherwise you can do it on a light table using a good magnifying loupe.
Actually, 35mm is quite easy to register because you can use the sprocket holes as registration guides.
Cutout the mask from the 4x5 film. On my D2 I leave a small tab on the right side to attach the film and cut the long sides at the top of the sprocket holes (film edge on the transparency). I have to do this to fit the carriers on the D2 which have small pins in them for film alignment - if you own or have seen the carriers you know what I mean.
Tape the mask emulsion side up on the lightbox using the edge of the small tab. Place the film emulsion side up on top of the mask.
Have several small pieces of the 3M tape pre-staged and hanging on one edge of the light box.
Fiddle with the film under the magnifying loupe until you are satisfied that the film is totally registered with all sprocket holes along both edges of the film.
Hold the film in place and tape the edge of the transparency to the small tab on the mask using a piece of the 3M tape.
At this point, you will have taped the film to the light box twice. Liberate the film & mask from the light box by using an X-Acto knife to cut the tape at the edges of the mask tab. The transparency will be hinged to the mask.
You now have a transparency with a mask. Clean both sides of the film & mask. Don't forget to clean between the transparency and the mask. I just use canned air and gently blow off any dust.
Put the transparency mask sandwich into the enlarger with the mask on top & the transparency on the bottom emulsion side down.
Make a test print on the Ilfochrome material and then start making printing adjustments for density, color, minor dodging/burning etc.