How can I make my scones rise evenly?
Your technique is going to be critical here.  Leaning scones aren't necessarily indicative of improper technique, but flat ones are.

Keeping your ingredients cold is important when creating scones in every recipe I've read or tried.  Cook's Illustrated went so far as to grate the butter and then freeze the grated pieces and use a laminating technique to provide layers.  Many other recipes use cold butter (frozen or simply very cold out of the freezer) in chunks and a pastry blender to cut them in.  Your milk should also be cold, not room temperature.  You may also want to chill your work bowl and utensils.  You don't want your butter melting before those scones hit the oven.

When your scones hit the oven and the butter does begin to soften and melt, it will leave behind layers of air in your scone which will help it to rise.

You don't want to handle finished scone dough very much.  Use a light touch and minimal work.

I'd also check the date on your self-raising flour.  The ingredients will lose their activity over time, causing the flour not to rise as well any more.