What do I do if I want to invest different amounts every month?

In that case, you can choose ‘park money safely’ and make a lump-sum investment now. You can repeat the same in future months.
However, in our experience, people rarely manage to sustain this tempo if they do it manually. It is much better to decide a minimum monthly amount and start an SIP for that amount. The amount then gets automatically transferred from your bank account each month.