There are broadly 3 stages to making a collage:
i) Assemble interesting paper
ii) Design
iii) Mount
1. Interesting paper includes:
a) “crash” paper in various colours.
Crashing ink & colour can be easily done on sized paper.
It can also be done on non-sized paper, but this needs to be thicker, strong enough to cope with getting saturated.  I like to use bark paper or mulberry paper.
The technique for colouring the paper is:
– mix dishes of the colours you want to use.  Not very thick but not too dilute: you want to be able to drip it and have a good intensity of colour.  If you are doing large sheets of paper, you need quite a bit of colour.  You can use mineral and/or vegetable colours: mineral colours tend to come out stronger.
– wash the paper with pale ink, using a large brush.
– drip or pour the colours on, but avoid pooling.  The colours can swirl together, but don’t mix them, or the overall effect will be muddy.  Leave it to dry.  If you pick the paper up when wet, it will run, so it is best to put it on several sheets of newspaper before you start, so that you can move it flat.
– do pieces of paper with different colours – some warmer, some cooler.
b) paper with calligraphy on.  This could be white paper, or crash paper.   You can use your sheets of calligraphy practice.  Try different sizes of calligraphy.  You could use this link as a starting point to find famous examples of calligraphy.
c) paper with painting on.  Again, white or crash.
For example I used pine twigs on one of my cards.
2. Design means selecting the pieces of paper to use, tearing or cutting them, and putting them together.   Include contrasting colours.  Design can include seals.
Qu Leilei is a master of collage, for example The Sun in My Dream
Here are a few of Leilei’s collages that I own, as examples.