Block Strings

Most special moves are unsafe on block so use normal attacks, and if you are using a special move, you can cover your self with an assist.

Solo Blockstring= Normal Attacks
Duo/Trio Blockstring= Normal Attacks + Specials + Assists
(Some specials like Sonic Homing or Shadow Teleport work for block strings if solo but aren't as safe as just using normals)

How to Use Assists

Combo Extension

Some assists are OTG which would allow a character like Shadow who gets limited OTG to do more optimal combos. So when after a move that sends a character down, use an OTG assist.
You could also time it with a non OTG assist.
After the assist finishes, continue the combo or continue attack during combo.

Extra Super Damage

Use an assist right before your supers for extra damage, assists scale differently to your main combo.

AS+ATAS+SAS

Assist Switch
Assist to Assist Switch.
Super Assist Switch
Special Assist Switch

Here is an example of these techniques I've named.
Assist Switch - Switch character to the character using assist.
Super Assist Switch - Bring in an assist before Sonic LVL 2 and once LVL2 is activated, assist switch
Special Assist Switch - Knuckles prolonged Seismic Toss to an assist switch. This resets your combo but the opponent will bounce letting a AS character do anything.
Assist to Assist Switch. - Mecha A + Knuckles B. Switch to Mecha and continue combo.

In Training

When using 2 controllers. Press hold an input and press the windows button.
Let go of your input.

You will see a character repeating that input.
If you press windows again you are in the game and the character is repeating the input.

You can use this to make a block dummy.
Or practice grabbing someone in the air.

I tried this with using DS4windows on windows without steam.
On Linux with Steam, it may be possible if you use the shortcut for steam overlay.
Alternatively just physically do this with a suitable item.