This is my method for outdoor slides with covering the most distance being the goal in mind. there are other variations of sliding.

ORDER OF INPUTS.

1. Hold W + A/D
   the strafing is only for increasing your run speed in-between slides

2. Swing the Black-Jack. do this while correcting you aim direction. you need to release A/D before you Lean forward. be looking down enough to mantle the blackjack.

3. Lean Forward. then almost immediately..

4. Side Lean and Jump Immediately afterward to mantle.

if you do it right you will slide forward a considerable distance and hear the mantle sound. don't hold down jump for too long or you will teleport back to where you started.

i use Left Alt for lean forward, with my thumb pressing both Alt then space (Jump) shortly afterward.

You can only begin leaning forward AFTER you have completely stopped pressing A or D. UN-strafing will cancel the lean forward. any strafe momentum will interrupt your forward lean, and it lingers slightly after you let go of A/D.

you can test this by walking forward, then leaning forward, and pressing A or D. The forward lean will be canceled. (or while strafing. lean forward, and let go of A or D and you will stop leaning)

Don't do either if you want a full distance slide. Strafing while doing a slide will usually result in a shorter slide, in the direction of your side lean. ( and is quite awkward to do )

TIMING THE MANTLE.

this will vary with your inputs and camera angle. Roughly where the end of the "swoosh" sound of the black jack, is where the timing window is. Just before it ends, the same time, or just afterward. varying with the amount of Lean and/or Camera angle

you can try it by just standing still and swinging the BJ, varying your view angle and jumping.

its somewhere between looking straight-down and definitely much lower than straight-ahead.

Don't practice on Running Int, you can see better on Framed. and you don't have to listen to Basso.

this is all based off my own experience and anyone trying to do it should verify each step for themselves. the inputs all happen within less than a second. i have no idea if screen res effects things, and obviously input lag could be an issue for some people.

update with a screen refresh rate higher than 60 ( max difference is at 100hz) you will see a slightly longer slide. but at the cost of the input timing being a lot smaller. ( its already small ) also, it has been discovered by Thxfor, that an outdoor slide is possible while aiming slightly up from straight ahead. this timing is different and also tighter than the OG method but also travels a bit further.