This is a bit advanced, so please check everything twice, it's possible to break things here!
Add a radio button or select list to your donation form. Now you can choose between several options or use "other" to customize it.
If you want to change the suggested amounts:
Edit the form, and in the "Amount" box you have a new field (probalby simply showing some number, the label doesn't show because its customized, but it shows to the end user), now click on the edit-button/crayon-icon of the new field:
The "Properties" tab gives you the title for the field and beneath it
"form_key", this one is important and must be set to "donation_amount".
The "Allow Other" button enables the other field, else you just have suggested amounts of your select list.
With the "Options" tab you can customize your suggested amounts.
Click on "Customize Keys" and change both numbers! This is important, otherwise the wrong amount gets charged! (the keys are the ones that count)
Check if the field is "required" at the "Validation" tab.