remove the 2 bolts for the stock and then the 2 bolts that hold the trigger onto the barrel. Remove the large cap on the back of the barrel with a large screw driver. Be careful to hold it in place when removing it because it is under spring pressure. Once removed, remove the firing pin, cap, and two springs. Then cocking lever should be loose now so it will come out. You can now use the breech tool to remove the nipple and breech plug. reverse to assemble.
I would need to know the brand and model, and I could help.
You need the services of a gunsmith
http://stevespages.com/pdf/marlin_15yn.pdf
Send me your email & I'll send you a scan of the manual.
Marlin 336 Owners Manual at Marlin Firearms website: http://www.marlinfirearms.com/pdfs/manuals/MFC_Centerfire.pdf
Follow this link to the Marlin website page for the 795, and you can download the manual for the weapon, which explains disassembly. http://www.marlinfirearms.com/Firearms/SelfLoading/795.aspx
On the main Marlin Firearms website, there is a section labeled "Owners Manuals", from which you can download the manuals of Marlin products in .pdf form. Assembly and disassembly instructions should be included in these manuals. See related link.
If you go to Marlin's web site you can download a pdf file of the manual, which includes instructions on how to disassemble your 1894. http://www.marlinfirearms.com/pdfs/manuals/MFC_Centerfire_1894.pdf
The first step should be to read the owner's manual. You can request a paper copy from marlin, or go the the link below that says MARLIN, and it will take you to the owner's manual. Check that the rilfe is unloaded before performing ANY maintenance.
If you don't have an owner's manual, find a gunsmith. You can try gun shows and want ads to find a manual as well as internet searches. Good luck.
They are called muzzleloaders simply because the powder and bullet are loaded from the muzzle end of the gun, Here is a page from WIKIpedia that may help answer.http://en.wikipedia.org/wiki/Muzzleloader
If this is a moderm manufacture gun, the company will provide an owner's manual which will have disassembly instructions. Contact them at their web site. If this is an antique gun, you are best advised to not disassemble it if you don't know what you are doing. You can damage it and significantly reduce its value if you take it apart incorrectly.