We are not going to concern ourselves (for now) with the traditional Welsh counting system (based on counting in twenties). There is a wiki entry giving a detailed explanation.
We are just going to look at how Welsh numbers from twenty to one hundred are formed. This is straight-forward once you have learned to count to ten (which you can do here) if you need to.
The 'modern' counting method is based on the decimal system, which we are all familiar with.
Lets start with dau ddeg 'twenty'. This has the literal meaning 'two ten'. The word deg 'ten' has changed to ddeg due to due to mutation.
Thirty is tri deg 'three ten'.
Forty is pedwar deg 'four ten'.
And so on.
The two numbers to look out for on our count to one hundred cant in Welsh are fifty and sixty. These are Pum deg and chwe deg respectively. Pump 'five' and chwech six have changed to Pum and chwe. They have mutated.
And what about the numbers in between?
Simply add the appropriate number from one to ten to your dau ddeg, tri deg ...
Let's use ninety nine as an example, which in Welsh is nine, ten nine.
Nawr deg nawr.
See also numbers 1 to 10, numbers 11 to 20, and telling the time.