Step 1) Sign up for a
MyBitcoin account.
Step 2) Go to
Merchant Tools.
Step 3) Use the button code generator to create a button. Leave it unencrypted.
Step 4) Copy the HTML code and paste it into your website.
Step 5) Change this
<input type="hidden" name="amount" value="1.00">
to this
<input type="text" name="amount">
This adds a payment button and a text box where the user can enter an amount. If you know HTML, you can change that to a select list or some other method of the user entering an amount.