Proper Introduction

I’m about to introduce my 70 pound female to a 50 pound male. Both are lab mixes, both altered. I’ve been conflicted on how I can make this introduction as successful as possible.

First, should I introduce them on a leash, or off leash? I have a large fenced in yard. Second, should I let her smell his blanket first?

The male dog I’m getting is currently scheduled to be euthanized soon. I want so badly for this to work. Thanks so much.