Jaime,
Yes, mate the negative onto the black side of the tissue. If you are doing single transfer, then if you mate the emulsion side to the tissue, the image will come out reversed on the final support. I only worry if there is lettering in the image that would reverse; in that case I mate the black side of the tissue to the non-emulsion side of the negative, although it does cause a slight loss of sharpness.
A wise precaution is to place the tissue black side up, then put a thin piece of plastic film (I use cling-film for wrapping sandwiches) and then place the negative. I find that just occasionally I may get a little bit of tissue that was not fully dry and if place directly on the negative, it may stick and cause significant swearing.
Put the stack of tissue and film into a contact frame to squash it all together and then expose to UV light. I used an old picture frame for a while - one of the cheap board/glass 'clip frames', but made a decent wood contact frame eventually to save on plasters for the cuts on my hands.
Best regards,
Evan