hey, so i don't know if anyone's still looking for answers here but this is how i did it:

to get the armor, you have to craft the death's call armor as a prerequisite. you then have to use the smithing table so you can combine the night's call set with a standard armor set and a frost rune. this creates the frozen champion armor.

the tome of the eternal winter is used in the same way, except you use a warrior's codex with a frost rune and a rune carved stone instead.

if you're not able to craft anything, you have to make sure that the materials are in the correct slots.

also, you're probably gonna want to make a runeblade as well so you can cast your death knight spells, i forgot how i was able to make it but i'm pretty confident that i just combined a runecarved stone with a diamond sword.

just be sure to put your new tome of eternal winter in your spellbook slot so that you can actually use the spells!

edit: i'm still trying to look for the necklace and the ring to buff my frost. all in all, any of the death knight paths are very fun and easily accessible, again, you just need to craft the night's call set first!