As I said earlier if your paper is 140lb (300gsm) or less you will need to stretch it onto a suitable base first. Not hard to do - you don't require any fancy equipment. Just a flat wooden board, a sponge, an old towel and Gummed paper packing tape. Oh a container of water, I use the one I paint with (a large plastic fizzy drink bottle with the top sliced off.
Lay the towel flat over the base board you're going to stretch the paper on.
Lay the paper onto the towel.
Tear off 4 pieces of the gummed paper tape, relative to the paper size i.e. obviously about an inch or so longer than each edge.
Soak the sponge in the water and liberally sponge both sides of the paper, doesn't realley matter how much water you use.. thearapy time
give the paper a good bath
Sandwich the watercolor paper between the towel, lightly press with your hands all over. perhaps paying a little extra attention to where the edge of the paper is between the towel.
Place the paper on the board, neatly fold the towel and place to one side as shown below.
With a pencil just place a couple of lines a 1/4 of an inch in from each edge. this isn't totally necessary it just acts as a guide when postioning the tape.
Using the wet sponge and folded towel pull the first piece of tape through using your finger and thumb... I usually pull do one half then turn it around and do the other half.
Now you'll see the point of the pencil marks,, you just align the tape to them then gentle smooth each piece of gummed tape to the four edges of the paper.
Just run a piece of kitchen paper or tissue all around the edge to tidy up. I very often run my thumb nail gently over the lip that froms between the papers edgeand the base board.
leave somewhere save ready to paint.. give it an hour or so and off you go..
If you can't wait you can use a hair dryer on low speed to dry the whole lot off. Paint in minutes.