its easy but can be tricky well,
when i was in a game it was wave 4 and i got down while re-loading my mg42 i got revived and had no weapon so i go to the box and get 3 weapons mg42 m1 garand and the ray gun so 2 wave later i try it any it works again , and so it does for my friend
although i have not been on der riese for about 10 hours ive heard thiers a patch laready been out so if it still works i dont know , try it and tell me 1 you think (i.e if it works or not)

note: i found this on my second game and i was 1 of the top 500 people to get the maps only 350 had played after we died so no 1 start saying they found it because its a lie! unless you found it your first game witch at like 11 am on the thirsday